The hypothesis that the ciliary locomotion of rotifers is size limited and that it accounts of a significant portion of the energy budget was investigated using the genera Brachionus and Asplanchna. Speed of movement was measured among clones of different size in Brachionus, which shows little size variation through development. The same tests were done among individuals of different size within a clone of Asplanchna, which shows significant postembryonic size increase. In both cases, relative speed (body lengths per second) decreased significantly as body size increased. On this basis, and ecologically limiting size for ciliary locomotion is proposed. The actual cost of locomotion was measured for Brachionus; it is 62% of total metabolism, even though the theoretical (calculated) power requirements are well below 1% of total metabolism. Ciliary locomotion in the Rotifera thus appears to be extremely inefficient (low ratio of theoretical to actual power requirements). This hypothesis is supported indirectly by the sensitivity of speed to total metabolic rate in Brachionus: both plateau over the temperature range 20-32°C and decline in parallel outside this range. Unexpectedly high actual cost of locomotion is proposed as an important disadvantage of the Rotifera, partly offsetting the advantages accruing to them from small body size.