Climatic data and collection records for the 'cactophilic Drosophila aldrichi' and 'Drosophila buzzatii' for 97 localities were used to examine the effects of geographical location, season, host plant species and climatic factors on their range and relative abundance. Temporal variation in relative abundance was assessed from monthly collections over 4 years at one locality. Effects of weather variables over the 28 days before each collection were examined. A generalized linear model of the spatial data showed significant geographical variation in relative abundance, and significant climatic effects, with the proportion of 'D. aldrichi' higher in the warm season, and increasing as temperature variation decreased and moisture indices increased. The temporal data gave generally concordant results, as D. aldrichi proportion was higher in summer and autumn, and increased as maximum and minimum temperatures increased, and as variation in maximum temperature decreased. In a laboratory competition experiment, 'D. aldrichi' eliminated 'D. buzzatii' at 31°C, but was itself eliminated at 18°C and 25°C. The range of 'D. buzzatii' is constrained only by availability of its host plant, Opuntia species, although its relative abundance is reduced in the northern part of its distribution. The range of 'D. aldrichi', from central Queensland to northern NSW, Australia, is entirely within that of 'D. buzzatii', and its relative abundance decreases from north to south. Both climate and weather, particularly temperature variability, have direct effects on the relative abundances of the two species, and both likely act indirectly by influencing the outcome of interspecific competition.