The effectiveness of immigrants as agents of gene flow was investigated in a laboratory model, using mutant marker strains of the flour beetle, Tribolium castaneum (Herbst). We show that immigrants had an advantage over residents. The proportion of hybrid offspring (PHO), resulting from immigrant mating with residents, was higher than expected from their frequency in the parental population. This advantage was observed regardless of immigrant sex and immigrant strain. The advantage seems to result from immigrant mating advantage (although not a rare-male phenomenon) and not from better survival of hybrid offspring. However, hybrid offspring seem to be more resistant to sporozoan infection, resulting in higher PHO in sporozoan-infected cultures.