- 1. Host-associated differentiation (HAD) may be an important driver of parasite biodiversity. The sympatric occurrence of host-associated populations features prominently in studies documenting HAD but this neither confirms nor denies that the lineages arose in sympatry. It does raise the question of what maintains such structure despite the proximity of populations in space and time. 2. We tested for immigrant inviability in yellow pecan aphid Monelliopsis pecanis Bissell, a species previously shown to be host associated, using a laboratory reciprocal transplant experiment on pecan and water hickory. 3. Immigrant inviability on the alternative host is present in yellow pecan aphid. Immigrants to both hosts were significantly less fecund than residents and their nymphs were significantly less viable. Future studies should establish the genetic and non-genetic (maternal effects) basis of immigrant inviability in yellow pecan aphid in order to be certain about its contribution to pre-mating reproductive isolation. 4. Since aphids are cyclic parthenogens, inviability of both immigrants and their offspring has the potential to contribute to total pre-mating reproductive isolation. This is different than the case of obligate sexually reproducing organisms where offspring viability is a post-mating reproductive isolating mechanism. We discuss the implications of this for estimating the contributions of immigrant and offspring inviability to total reproductive isolation in aphids. 2011 The Authors. Ecological Entomology 2011 The Royal Entomological Society.