Ichthyophthirius multifiliis Fouquet (Ich) is a fish parasite that causes serious economic loss for aquaculture. A major difficulty in the maintenance of single isolates of Ich for research purposes is the loss of infectivity. After an unknown number of passages or infection cycles the Ich isolate loses its infectivity. This study determined the infectivity of an Ich isolate during 105 infection cycles in channel catfish Ictalurus punctatus over a 2 yr period. The mean percentage of fish infected by Ich, the infection levels and the time to trophont emergence were each compared after 4 cyclic periods: 1-25, 26-60, 61-90 and 91-105 Ich cycles. Results of this study demonstrated that Ich was significantly more infective (p < 0.05) at 1-25 than 26-105 cycles. Channel catfish were infected at a ratio of 1 infected fish to 8 naive fish at 1-25 and 26-60 cycles. A higher infection ratio occurred at 61-90 and 91-105 cycles. Trophont emergence was noted to be significantly longer at 91-105 compared to 1-25 cycles, during 7 and 5 d respectively, at 23.4 +/- 1.1 degrees C. The results of the present study indicate that the infectivity of I. multifiliis started to decrease after 25 infection cycles and was predominant in the single Ich isolate at 61-90 and 91-105 cycles.