Arginine is an essential nutrient for the proliferation of Toxoplasma gondii parasites. We have shown previously that parasites encounter different arginine concentrations as they infect different organs of their host, and that parasites respond by tightly controlling arginine uptake. They do this through regulating the expression of the major plasma membrane-localised arginine transporter, TgApiAT1. In arginine-replete conditions, TgApiAT1 expression is decreased, whereas TgApiAT1 abundance increases when arginine concentrations are lowered. Arginine-dependent TgApiAT1 regulation is mediated post-transcriptionally by an upstream open reading frame in the TgApiAT1 transcript. To better understand how parasites sense and respond to arginine availability in their environment, we have developed an arginine-sensitive fluorescence-based reporter strain of T. gondii. Using this strain, we have undertaken a flow cytometry-based, genome-wide positive selection screen to identify genes important for arginine-dependent regulation of TgApiAT1. This screen identified a candidate nucleolar RNA methyltransferase as being important for repressing TgApiAT1 expression when arginine is abundant. The screen also identified a putative RNA-binding protein and protein that may prevent ribosome stalling as being important for upregulation TgApiAT1 in arginine-limiting conditions. We are now developing approaches to understand the molecular mechanisms of how these proteins function in mediating arginine-dependent expression of TgApiAT1. Overall, our findings enhances our understanding of how a model intracellular pathogen responds to nutrient availability in its environment.