Stromata of grass-infecting fungi from the genus Epichloë (Clavicipitaceae: Ascomycota) serve as a food source and egg-laying surface for flies of genus Botanophila (Diptera: Anthomyiidae). Larger stromata should make it possible for flies to lay more eggs and provide more food to offspring. This hypothesis was tested in four different grass—fungus associations that occur in central Poland. In two of these associations, Epichloë bromicola on Elymus repens and Epichloë typhina on Puccinellia distans, flies showed a preference for longer stromata, and egg density on these stromata was significantly higher than in the other two associations.Anegative correlation between egg density and offspring success was observed in only one association, E. bromicola-El. repens. However, offspring success in this association did not differ significantly from offspring success in associations with lower egg density on the stromata, in which flies showed no preference for the stroma length. Long-term observations (2000–2010) of fly—fungus interaction in the E. typhina—P. distans association showed that fly preference toward stroma length may vary over time but with no clear tendency. No significant correlations were found between the larval density on a stroma and either larval weight or mortality. The results of the current study question our assumptions that egg laying depends on the stroma length and the fate of eggs laid (i.e., their hatching success and the condition, in terms of weight and survival, of the larvae) on egg density. It is possible that flies choose stromata based on attributes other than size.