Two field experiments were conducted from 1993–1994 through 1995–1996 growing seasons in Harney County, OR, to determine the relative competitive abilities of Elymus elymoides (squirreltail) a native perennial range grass, and Taeniatherum caput-medusae (medusahead), an exotic annual grass weed. The 1993–1994 growing season was very dry, 1994–1995 was dry, and 1995–1996 was wetter than average. One experiment tested seedlings vs. seedlings in each of three seasons. The second experiment tested seedlings plus second- and third-year established E. elymoides plants vs. 77 caput-medusae over 2 yr. Biomass, seed production, and soil moisture utilization 15, 30, 45, and 60 cm deep by the two species were measured. A randomized block design with factorial arrangement was used, with 25 2.25-m2 plots per block. Initial seeding densities of each species were 0, 10, 74, 550, and 4,074 seeds m−2 in all combinations of density. In the seedling vs. seedling experiment, intraspecific competition by 77 caput-medusae on itself was always significant (P ≤ 0.10) for both biomass and seed production. Interspecific competition by E. elymoides seedlings on T. caput-medusae biomass and seed production was not significant (P ≥ 0.10) in 2 of 3 yr and was always less than intraspecific competition by 77 caput-medusae. Only 0.4% of E. elymoides seed germinated, and no seed was produced in the very dry first year, but 84% of remaining seed was viable for the next year, which had better moisture conditions for germination and establishment. Interspecific competition affected (P ≤ 0.10) E. elymoides seedling biomass and seed production throughout the study. Intraspecific competition affected (P ≤ 0.10) seedling E. elymoides seed production in the dry year but not in the wetter than average year. In the mature E. elymoides experiment, intraspecific competition by T. caput-medusae on weight and seed production per plant was greater than interspecific competition from E. elymoides. Seedling/mature E. elymoides reduced T. caput-medusae weight per plant in the dry year but the effect was not biologically significant. Larger, mature Eelymoides plants produced 600 to 3,000 seeds per plant during the wet year; neither intra- nor interspecific competition was a factor. Taeniatherum caput-medusae was better able to access deeper soil moisture and was more aggressive at extracting soil moisture than were E. elymoides seedlings in the wet year. Cold soils and low oxygen due to wet soils may have restricted E. elymoides seedling root activity. Mature E. elymoides plants did not appear restricted by cold soils or low oxygen. Established second- and third-year E. elymoides plants were able to compete for soil moisture down to 45 cm. The generally greater interspecific competitive effects of T. caput-medusae on E. elymoides than vice versa suggested that it will be difficult to establish an E. elymoides stand in an existing T. caput-medusae community without first suppressing T. caput-medusae. Individual E. elymoides plants did establish and were productive with and without T. caput-medusae competition.