This study investigates the oscillatory behavior of dynein arms in eukaryotic flagella. Using optical trapping nanometry, researchers measured the force generated by dynein arms on isolated doublet microtubules. When activated by photolysis of caged ATP, dynein arms generate a peak force of approximately 6 pN and move the singlet microtubule over the doublet microtubule in a processive manner. The force and displacement oscillate with a peak-to-peak force of ~2 pN and amplitude of ~30 nm. The geometry of the interaction suggests that very few (possibly one) dynein arms are needed to generate the oscillation. The maximum frequency of the oscillation at 0.75 mM ATP is ~70 Hz, decreasing as ATP concentration decreases. Similar oscillatory forces are generated by inner dynein arms on doublet microtubules depleted of outer dynein arms. These findings suggest that oscillation is an inherent property of dynein arms and may be a fundamental mechanism underlying flagellar beating.This study investigates the oscillatory behavior of dynein arms in eukaryotic flagella. Using optical trapping nanometry, researchers measured the force generated by dynein arms on isolated doublet microtubules. When activated by photolysis of caged ATP, dynein arms generate a peak force of approximately 6 pN and move the singlet microtubule over the doublet microtubule in a processive manner. The force and displacement oscillate with a peak-to-peak force of ~2 pN and amplitude of ~30 nm. The geometry of the interaction suggests that very few (possibly one) dynein arms are needed to generate the oscillation. The maximum frequency of the oscillation at 0.75 mM ATP is ~70 Hz, decreasing as ATP concentration decreases. Similar oscillatory forces are generated by inner dynein arms on doublet microtubules depleted of outer dynein arms. These findings suggest that oscillation is an inherent property of dynein arms and may be a fundamental mechanism underlying flagellar beating.