Theronts of the ciliate Ichthyophthirius multifiliis (Fouquet) with up to 4 micronuclei were recorded in populations from two different parasite isolates, maintained as primary infections in juvenile carp through 2 and 10 cycles, respectively. The largest number of multimicronucleate forms occurred within the older isolate (10 cycles) following cyst incubation at 20"C. Tro-phonts were induced to emerge from the host epidermis following incubation of the fish in Eagles MEM for 10-15 min at 20"C. This provided for the first time a technique to recover trophonts in all stages of development; observations were made on the transition from theront to trophont, with respect to nuclear events, the organelle of Lieberklihn, and phagocytosis of host cells. On re-exposure of carp already carrying a two-day primary infection, recently entered trophonts were found in various stages of fusion with established parasites. Results are discussed with respect to conjugation, anisogamy and senescence.