1. The marine snail, Littoraria filosa, lives on the leaves and trunks of mangroves. Populations of L. filosa are annual when its dipteran parasitoid Sarcophaga megafilosia is present, but perennial in its absence.
2. The size distribution of naturally occurring parasitised L. filosa suggests S. megafilosia has a distinct preference for snails with shells ≥ 10 mm long but, paradoxically, most individuals ≥ 18 mm long disappear from annual populations in late spring/early summer without appearing in the parasitised cohort.
3. When S. megafilosia was offered an unrestricted size range of L. filosa, only individuals >10 mm long were attacked and those from 19 to 20 mm long were preferred.
4. In no-choice experiments, L. filosa <10 mm long were parasitised but pupation did not occur. For L. filosa >23 mm long, most attacked snails succeeded in ejecting the parasitoid larva but often lost their grip on the wall or ceiling of the cage and fell. Littoraria filosa is unlikely to survive such dislodgement from mangrove trees.
5. These results can explain the different size distributions of live and parasitised snails in the field, including the disappearance of large snails that do not subsequently appear in the parasitised cohort. However, this does not explain the preference by S. megafilosia for L. filosa around 19–20 mm for which more than 50% of attacks failed. This preference may be also determined by increased parasitoid fecundity and longevity as snail size increases.
6. If small differences in host size affect parasitoid fitness, it may help explain why attempts at biological control using known parasitoids of an exotic species are often ineffective or unsuccessful.