Life history and production of aDeleatidiumspecies (lilligroup) (Ephemeroptera: Leptophlebiidae) was studied for a 12‐month period in a flood‐prone river in the South Island, New Zealand in 1985–86. Mean larval density was highest (9170–18 580 m2) following a long period (> 5 months) of low stable flow (< 30 m3s−1), but typically ranged from 3550 to 4310 m−2. The annual cycle ofDeleatidiumwas difficult to interpret but there appeared to be two overlapping, poorly synchronised generations each year. Production of the winter generation calculated using the Size‐frequency method (51.40 g DW m−2), was five times higher than that of the summer generation (9.17 g DW m2) and annual production was 60.57 g DW m−2. Mean biomass of the winter generation (7.59 g DW m−2) was higher than that of the summer generation (0.87 g DW m−2) but the production to biomass ratio (P/B) of the summer generation (10.54) exceeded that of the winter generation (6.77). The ability to feed on low‐biomass epilithic layers, behavioural responses that minimise population losses during floods, and flexible life history patterns enable this species to attain high annual production in a physically harsh environment.