AbstractCopepodids ofCaligus epidemicussurvived for up to 7, 8 and 5 days at 19, 22 and 26 °C, respectively, without food. Activity was high up to 3 days and then declined, but 3‐ or 4‐day‐old copepodids were more infective than younger copepodids, suggesting an initial dispersal phase. Although the success of attachment to perchlets,Ambassis marianus, was higher in small volumes of water, there was no statistically significant association between water volume (1, 4, 60 and 2001) and success of attachment. Most copepodids and early chalimus stages in single and multiple infections were attached to the pectoral fins, but did not settle preferentially beside other copepodids on the skin of the host. The parasite was overdispersed (negative binomial distribution) when groups of fish were exposed to 100 copepodids. Perchlets were killed when exposed to large numbers of copepodids and adultC. epidemicus, but survived for at least 5 days after exposure to 25 or fewer copepodids and five or fewer adults (although not necessarily all att