Males of the Neptune whelk, Neptunea arthritica, prefer mated to non-mated females. However, this mate choice strategy could be deemed atypical, since under sperm competition, males are selected to exercise prudence in mate choice. To investigate how mate choice affected the outcome of sperm competition contests, paternity were determined among the offspring of 13 triple mated females from male mate choice trials; using polymorphic microsatellite markers. When presented with the choice between non-mated and mated females, males chose mated females consistently. Out of the 13 whelk families analysed, three resulted in triple paternity, seven resulted in dual paternity and three resulted in single paternity. The third males to mate sired 60% or more of the offspring in 11 out of the 13 families. Difference in paternity shares between first, second and third males to mate with a female seemed to be associated with the intermating interval and occurrence of “loose sperm” during mating. The present results suggest that, regardless of females’ multiple mating, males can still achieve considerable reproductive success allowing for an opportunistic bias towards copulated females at short intermating intervals.