Two spontaneous mutations of rudimentary, the gene encoding the first steps of de novo pyrimidine biosynthesis in Drosophila, are suppressed by mutant alleles of the suppressor of Hairy-wing locus. This interaction differs from typical su(Hw) suppression in that neither rudimentary allele is associated with an insertion of the gypsy retrotransposon. One allele, rsP1, appears to be a point mutation. Adult rsP1 homozygous females accumulate substantially less 7.3 kb rudimentary transcript than do wild-type females. The other allele, rsP2, is an insertion of an mdg3 retrotransposon in the sixth exon of rudimentary and in the opposite transcriptional orientation. This insertion divides the rudimentary locus into two separate, yet functional, transcription units by truncating transcription from the rudimentary promoter and promoting transcription of downstream rudimentary sequences. Phenotypic suppression of both rsP1 and rsP2 by mutant alleles of the suppressor of Hairy-wing locus correlates with enhanced levels of the rsP1 and rsP2 transcripts.