Spontaneous mutations were allowed to accumulate over 209 generations in more than 100 lines, all of them independently derived from a completely homozygous population of Drosophila melanogaster and subsequently maintained under strong inbreeding (equivalent to full-sib mating). Traits scored were: abdominal (AB) and sternopleural (ST) bristle number, wing length (WL) and egg-to-adult viability (V). On two occasions--early (generations 93-122) and late (generations 169-209)--ANOVA estimates of the mutational variance and the mutational line x generation interaction variance were obtained. Mutational heritabilities of morphological traits ranged from 2 x 10(-4) to 2 x 10(-3) and the mutational coefficient of variation of viability was 0.01. For AB, WL and V, temporal uniformity of the mutational variance was observed. However, a fluctuation of the mutational heritability of ST was detected and could be ascribed to random genotype x environment interaction.