Adults of the isopod Paragnathia formica inhabit burrows in saltmarsh banks from which they release larvae during autumn high tides. Larvae pass through three moult stages, each of which feeds ectoparasitically on estuarine fish (including Pomatoschistus microps), before a final moult to a non-feeding adult stage. The entire energetic reserves for survival and reproduction up to nine months (females) or 16 months (males) later are therefore acquired during these three brief periods of parasitism. Application of a plankton sampling technique showed larval density in the water to vary considerably between successive high tides. High densities of larvae (1 per 1·2 l) occurred on only one tide during a week-long study, when a corresponding peak in parasite prevalence in the fish population (10%) was recorded. Peak larval release was observed as the tide rose, at a time when host fish have been reported to be in greatest abundance. Considerable larval longevity was demonstrated in the laboratory in the absence of food (mean=43 d). Data are interpreted, in conjunction with field observations, in relation to larval parasitism opportunities post-birth.