The frequency of crossing over between two sporidial color mutations, between mutant sites in the white (w) locus and the centromere, and between two complementing mutations in the w locus was determined by teliospore colony (intact tetrad) analysis. Recombinant chromosomes were frequently included in disomic basidiospores as products of a meiotic primary nondisjunction. Failure to terminalize chiasmata originating in centromere-proximal chromosomal regions may be the basis of the nondisjunction observed. A qualitative method for sequencing mutant sites in the w locus is presented.