Missense mutations at the three hotspots in the guanosine triphosphatase (GTPase) RAS—Gly ¹² , Gly ¹³ , and Gln ⁶¹ (commonly known as G12, G13, and Q61, respectively)—occur differentially among the three RAS isoforms. Q61 mutations in KRAS are infrequent and differ markedly in occurrence. Q61H is the predominant mutant (at 57%), followed by Q61R/L/K (collectively 40%), and Q61P and Q61E are the rarest (2 and 1%, respectively). Probability analysis suggested that mutational susceptibility to different DNA base changes cannot account for this distribution. Therefore, we investigated whether these frequencies might be explained by differences in the biochemical, structural, and biological properties of KRAS ^Q61 mutants. Expression of KRAS ^Q61 mutants in NIH 3T3 fibroblasts and RIE-1 epithelial cells caused various alterations in morphology, growth transformation, effector signaling, and metabolism. The relatively rare KRAS ^Q61E mutant stimulated actin stress fiber formation, a phenotype distinct from that of KRAS ^Q61H/R/L/P , which disrupted actin cytoskeletal organization. The crystal structure of KRAS ^Q61E was unexpectedly similar to that of wild-type KRAS, a potential basis for its weak oncogenicity. KRAS ^Q61H/L/R -mutant pancreatic ductal adenocarcinoma (PDAC) cell lines exhibited KRAS-dependent growth and, as observed with KRAS ^G12 -mutant PDAC, were susceptible to concurrent inhibition of ERK-MAPK signaling and of autophagy. Our results uncover phenotypic heterogeneity among KRAS ^Q61 mutants and support the potential utility of therapeutic strategies that target KRAS ^Q61 mutant–specific signaling and cellular output.