Abstract: The effect of early-spring submergence on larvae of a wetland-inhabiting population of the butterfly Neptis rivularis (Scopoli, 1763) (Nymphalidae: Neptini) was investigated in laboratory. Wild-collected, late hibernation larvae originating from inundated marshes of the Trebon Basin (Czech Republic) were exposed to two conditions possibly experienced during the floods, aerated (as under running water) and anoxic (as in still water). Submergence for up to three days did not affect larval survival; longer submergence caused substantial mortality, no larva survived the longest submergence tested (21 days). Since there were no differences between aerated and anoxic treatments, the mortality seemed to be due to a failure of osmotic balance. Very short submergence thus does not influence larval survival in field, but prolonged floods may substantially reduce population size. However, the extent of the floods varies in the years, there are some upland colonies of N. rivularis in the area, and the butterfly is a relatively good disperser. It is thus assumed that the species compensates for inundation-caused mortality by a re-colonisation of its sites.