This research examines the life cycle of the parasitic cymothoid isopod Glossobius hemiramphi and its role as a symbiont with its fish host, ballyhoo (Hemiramphus brasiliensis). Samples of H. brasiliensis were collected from July 1997 to October 1998 in nearshore waters of south Florida. Fish were randomly culled from a commercial lampara net fishery, and independent of the fishery, fish were collected with cast nets or hook and line. The average parasite prevalence was 10.1%, observed in 2,928 fish collected year round. Monthly prevalence ranged from 4.6 to 18.2% and was highest in the summer on small young-of-the-year fish. Prevalence declined with fish size from a high of 38.0% for fish 11-16 cm to a low of 3.3% for fish 28-29 cm. Ovigerous females were observed throughout the year and no within-brood mortality was evident. Marsupiumites developed through five distinct ontogenetic stages, and the final marsupial stage (manca) was likely immediately infestive upon release. Diminutive males (2.8-12.4 mm) were attached to the host's gill arches, and larger females (16.9-35.6 mm) occupied the buccal cavity. Only a single fish older than age-1 was infested. These results indicate that Glossobius hemiramphi is a protandric hermaphrodite with an annual life cycle. There was no evidence of a parasitic effect on the host fish condition (weight-length), but we cannot exclude the possibility that infested fish have a higher mortality rate than uninfested fish, at least temporarily (∼1 year).