Most life history traits are positively inﬂuenced by body size, while disadvantages of large size are poorly documented. To investigate presumed intrinsic costs of large body size in yellow dung ﬂies (Scathophaga stercoraria; Diptera: Scathophagidae), we allowed larvae from replicate lines artiﬁcially selected for small and large body size for 21 generations to compete directly with each other at 20°C (benign) and 25°C (stressful) and low and high food (dung) availability. Greater mortality of large line ﬂies was evident at low food independent of temperature, suggesting a cost of fast growth and/or long development for genetically large ﬂies during larval scramble competition under food limitation. Our results are congruent with a previous study assessing mortality when competing within body size lines, so no additional mechanisms affecting scramble or contest behavior of larvae need be invoked to explain the results obtained beyond the costs of longer development and faster growth. Thus, artiﬁcial selection producing larger yellow dung ﬂies than occur in nature revealed some, albeit weak mortality costs of large body size that otherwise might have remained cryptic. We conclude, however, that these costs are insufﬁcient to explain the evolutionary limits of large body size in this species given persistently strong fecundity and sexual selection favoring large size in both sexes.