X-ray-induced mitotic recombination within the lozenge locus of Drosophila melanogaster was found to be non-reciprocal. Recombination was detected by the presence of faceted spots in a smooth (lozenge) eye. Markers were present so that if recombination were reciprocal, equal numbers of vermilion and apricot-vermilion-coloured faceted spots would be formed. Of 19 faceted spots found among more than 60000 eyes examined, 10 were vermilion, 7 variegated (some vermilion and some apricot-vermilion-coloured facets), and 2 probably variegated. None was uniformly apricot-vermilion in colour. Four mechanisms for the formation of faceted spots are discussed: double reciprocal recombination, misrepair or gene conversion before chromatid replication, misrepair or gene conversion after chromatid replication, and misrepair or gene conversion followed by chromosome loss. The evidence most strongly supports the hypothesis that all faceted spots were formed by either misrepair following X-irradiation or gene conversion and were originally vermilion in colour. Subsequent chromosome loss from these vermilion cells led to the production of variegated spots. Variegated spots were generally smaller than those uniformly vermilion in colour, indicating that chromosome loss may occur with greater frequency in older irradiated larvae.