trichous reproduced well at 15 oc the chosen temperature for continuous culture. Primarily a bottom feeder, this ciliate consumed a sufficient quantity of standard food organisms to prevent detrimental accumulation. Reduction in bacterial population was demonstrated in small flasks, and no bacterial slime was observed in experimental copepod cultures. Eggs of Acartia rest singly on the bottom of the culture vessel until hatched. Although the ciliates fed in the same vicinity, they apparently did not affect normal hatching. Dead copepods quickly became engorged with bacteria, and ciliates could be seen crawling over their exoskeletal surfaces, apparently consuming bacteria liberated from the carcasses. Acartia fed on ciliates as well as on phytoplankton and thus limited ciliate population and completed the nutrient cycle. Euplotes vannus is now included routinely in all continuous cultures of Acartia spp. at this laboratory