A computer simulation model demonstrating the effect of directional selection upon a simplified quantitative genetic character, is presented. Complementary homozygous parents were crossed to generate a fully heterozygous F1 population. Phenotypic truncation selection was effected upon the F2 and subsequent generations, and genotypic and phenotypic response to selection, as well as the change in population heterozygosity, was monitored to generation F9. The model simulates selection for alternative breeding regimes, modes of allelic interaction, selection pressure, population size and levels of environmental variation. Three alternative methods for the simulation of the environmental component of phenotypic variance are incorporated, and mechanisms and relative merits of the rival methods are discussed. Data generated by the model conformed closely to theoretical expectation, lending support to the use of the model as a platform for development of more comprehensive simulation models. Potential improvements to the model are discussed.