Both intra- and interspecific interactions between microbes are likely to play an important role in determining the severity of microbial infections. Here, we study the impact of interactions between coinfecting opportunistic pathogens Staphylococcus aureus and Pseudomonas aeruginosa on both phenotypic and genetic changes in a P. aeruginosa social trait, the production of iron-scavenging siderophores. Siderophores are facultatively upregulated in response to iron limitation and play a key role in determining the virulence of microbial infections. Siderophore production is metabolically expensive to individual producers but benefits the group as a whole because siderophores can be used by all cells in the vicinity with siderophore receptors. Hence, populations of siderophore producers can be invaded by nonproducing cheats. Previous work has shown that P. aeruginosa can lyse S. aureus, supplying a source of free iron. We therefore hypothesized that the presence of S. aureus might result in facultative downregulation of siderophore production, and in turn, reduced selection for siderophore cheats. We tested this hypothesis by evolving P. aeruginosa in the presence and absence of free iron and S. aureus, in a fully factorial design. Iron had the expected effect: siderophore production was downregulated and cheats evolved less readily, but the presence of S. aureus instead increased facultative siderophore production and selection for cheats. This is probably because the S. aureus had the net effect of competing for iron, rather than acting as an iron source. This study demonstrates that interspecific competition can have a marked effect on intraspecific social interactions.