Some apicomplexan parasites, including Toxoplasma gondii and Plasmodium falciparum, replicate within parasitophorous vacuoles (PVs) after invading the host cell. Amino acids and other metabolites that are essential for parasite growth are delivered to the PV by transport through its membrane, which is made selectively permeable to small molecules by an unknown mechanism. Gold et al. now identify a novel T. gondii protein, GRA17, that localizes to the PV, and they propose that it forms a metabolite-permissive pore. Parasites lacking GRA17 were defective in small molecule transport across the PV membrane and were impaired in growth and virulence. Notably, these phenotypes could be rescued by the P. falciparum protein EXP-2, which is a component of the PTEX (Plasmodium translocon of exported proteins) complex. The complementarity between GRA17 and EXP-2 suggests a new function for EXP-2 in the transport of small molecules across the PV membrane.