ABSTRACT The authors report on the seasonal fluctuation of the poecilostomatoid copepod Modiolicola insignis associated with Mytilus galloprovincialis, living in Lake Faro (Messina). The copepod has four generations a year, seasonally succeeding each other. The cycle that takes place during summer has the highest number of individuals. By examining the sex ratio, we noticed a prevalence of males from May to August and of females from October to April, with the exception of November. It is possible to find ovigerous females as well as males all year. The largest number of females corresponds to that of males, occurring a month earlier than that of copepodids.