Previous work has shown that purified double mutant A204C/C374A yeast actin is polymerization-deficient in vitro under physiological concentrations. To understand the importance of the 204 residue in subdomain 4, a series of actin proteins with a single mutation at this position were created with Cys-374 retained. Only yeast expressing A204G-, A204S-, or A204C-actin were viable. The A204G and A204S strains were sensitive to cold temperature and hyperosmolarity, whereas the A204C strain showed more profound effects on growth under these conditions. Cells expressing A204C-actin exhibited anomalies previously observed for A204C/C374A actin, including abnormal actin structures. A204G- and A204S-actin proteins had 12- and 13-fold increased critical concentrations, respectively, relative to wild-type. Only at very high concentrations could A204C actin polymerize when ATP was bound; when hydrolyzed, the ADP-containing A204C filaments depolymerized, demonstrating a profound difference in critical concentration between ATP and ADP states with A204C actin. A correlation between size of the residue substituted at position 204 and energy minimization of actin filament models was observed. We propose that the region surrounding residue 204 is involved in interactions that change depending on the phosphorylation state of the bound nucleotide that might reflect different conformations of F-actin subunits.