1. Dark-field photographic records of the wave patterns of moving flagella have been made using multiple-flash exposures at flash rates of up to 50 per sec. Patterns obtained from ATP-reactivated isolated flagella show reduced amplitude of bending, but are otherwise similar to those obtained from flagella under normal conditions. The co-ordination required to produce propagated waves of active bending appears to be preserved after isolation and reactivation.

2. Addition of methyl cellulose to the medium to increase the viscosity reduces the frequency of beat much more than the amplitude. This behaviour can be partially explained by an analysis of the equations for sinusoidal wave movement of flagella which shows that maximum efficiency of forward swimming will be obtained if the amplitude of beat is maintained greater than ½π times the wavelength, and variations in available power or viscosity are compensated by changes in beat frequency.

3. Wave patterns at low frequencies in low ATP concentrations are unlike those obtained when the frequency is reduced by increased viscosity. The effect of ATP concentration on beat frequency is not explained by an effect on the power available for beating or by an effect on the ‘internal viscosity‘ of the flagella.

Much of the work reported here was carried out in the Department of Zoology at the University of Minnesota, with support from the United States Public Health Service (RG-7208) and the Graduate School of the University of Minnesota. At the California Institute of Technology the work has been supported by the United States Public Health Service (RG-6965) and the Max C.Fleischmann Foundation of Nevada.