Elaphognathia cornigera is a temporary fish-parasitic isopod crustacean that inhabits intertidal sponges. The life cycle and growth rate of this species were investigated using a laboratory rearing experiment. In addition, a wild population of this gnathiid isopod was sampled at fortnightly intervals over a 2-year period on an intertidal rocky shore at Oura Bay, Izu Peninsula, southern Japan. The population dynamics of this species were assessed using data from the laboratory experiment. Under laboratory conditions, individuals reached maturity after alternating between the parasitic feeding stage and the independent moulting stage three separate times. The mean larval period at 25°C was around 52 days. All life stages were observed in the natural population between spring and autumn. The density curves for all larval stages showed three to four distinct peaks, with the exception of the first larvae just after eclosion. According to the emergence patterns and growth rates derived from the laboratory experiment, E. cornigera appeared to alternate generations three to four times per year. During the winter months, only larvae in the second post-feeding phase and adult males were observed. This may have resulted from a halt in the development of larvae at the second post-feeding stage, triggered by cold winter conditions, and the post-reproductive death of semelparous females. Overwintered larvae and adult males became the source of the new population the following spring.