Complementary chromatic adaptation in the filamentous blue-green alga *Fremyella diplosiphon* involves changes in pigmentation and morphology in response to different light conditions. Cultures illuminated with red light show significantly less C-phycoerythrin (PE) and shorter filament lengths compared to those under fluorescent light. PE is a photoinducible component, and its synthesis can be initiated by transferring red-illuminated cultures to fluorescent light. Conversely, transferring fluorescent-illuminated cultures to red light stops PE synthesis. PE levels decrease in red-light adapted cultures due to transcellular breakage, not degradation. Light regulates PE levels by controlling synthesis rates. The study used spectrophotometric and immunochemical methods to quantify PE metabolism in *F. diplosiphon*. Cultures were grown under fluorescent or red light, and PE synthesis was studied using pulse-chase experiments with [14C]-phenylalanine. Fluorescent light induces PE production, while red light inhibits it. PE is stable under continuous fluorescent illumination, but its levels decrease in red-light adapted cultures due to filament fragmentation. The PE/PC ratio in red-light adapted cultures is about 20% of that in fully adapted fluorescent-light cultures. The morphological differences between *F. diplosiphon* adapted to fluorescent and red light are significant. Fluorescent-illuminated cultures have longer filaments and more cells per filament, while red-light adapted cultures have shorter filaments and fewer cells. These changes are fully reversible with growth. The study also demonstrated that PE is not degraded in continuous fluorescent light, but its levels decrease in red-light adapted cultures due to filament fragmentation. The results show that PE is a stable molecule under continuous fluorescent illumination, but its levels decrease in red-light adapted cultures due to transcellular breakage. The PE/PC ratio in red-light adapted cultures is about 20% of that in fully adapted fluorescent-light cultures. The study also showed that PE specific activity decreases when PE is synthesized de novo, but remains constant when synthesis stops. The mean filament length reduction in red-light adapted cultures is accompanied by decreases in PE and total incorporated radioactivity. These changes are fully reversible with growth. The study provides evidence that transcellular filament breakage occurs across necridia, leading to cell lysis. The results indicate that light influences PE levels by regulating synthesis rates, and that PE is a stable molecule under continuous fluorescent illumination.Complementary chromatic adaptation in the filamentous blue-green alga *Fremyella diplosiphon* involves changes in pigmentation and morphology in response to different light conditions. Cultures illuminated with red light show significantly less C-phycoerythrin (PE) and shorter filament lengths compared to those under fluorescent light. PE is a photoinducible component, and its synthesis can be initiated by transferring red-illuminated cultures to fluorescent light. Conversely, transferring fluorescent-illuminated cultures to red light stops PE synthesis. PE levels decrease in red-light adapted cultures due to transcellular breakage, not degradation. Light regulates PE levels by controlling synthesis rates. The study used spectrophotometric and immunochemical methods to quantify PE metabolism in *F. diplosiphon*. Cultures were grown under fluorescent or red light, and PE synthesis was studied using pulse-chase experiments with [14C]-phenylalanine. Fluorescent light induces PE production, while red light inhibits it. PE is stable under continuous fluorescent illumination, but its levels decrease in red-light adapted cultures due to filament fragmentation. The PE/PC ratio in red-light adapted cultures is about 20% of that in fully adapted fluorescent-light cultures. The morphological differences between *F. diplosiphon* adapted to fluorescent and red light are significant. Fluorescent-illuminated cultures have longer filaments and more cells per filament, while red-light adapted cultures have shorter filaments and fewer cells. These changes are fully reversible with growth. The study also demonstrated that PE is not degraded in continuous fluorescent light, but its levels decrease in red-light adapted cultures due to filament fragmentation. The results show that PE is a stable molecule under continuous fluorescent illumination, but its levels decrease in red-light adapted cultures due to transcellular breakage. The PE/PC ratio in red-light adapted cultures is about 20% of that in fully adapted fluorescent-light cultures. The study also showed that PE specific activity decreases when PE is synthesized de novo, but remains constant when synthesis stops. The mean filament length reduction in red-light adapted cultures is accompanied by decreases in PE and total incorporated radioactivity. These changes are fully reversible with growth. The study provides evidence that transcellular filament breakage occurs across necridia, leading to cell lysis. The results indicate that light influences PE levels by regulating synthesis rates, and that PE is a stable molecule under continuous fluorescent illumination.