In gene mapping, the genetic distance between two genes is measured by the frequency of meiotic crossovers occurring between them. It is generally assumed that there is more than one crossover per chromosome, and the distribution of crossovers along chromosomes is random and follows a Poisson distribution (no interference), or that interference is inversely correlated with distance. Although those assumptions may be correct for some organisms, we report here a novel exception in the marine mollusc Mulinia lateralis Say. Using segregation analysis of gynogenetic diploids, we found surprisingly high gene-centromere recombinant frequencies for most of the 13 allozyme loci studied. For at least six loci, there was always one and only one crossover occurring between the gene and its centromere, suggesting complete interference where the occurrence of one crossover completely suppressed the occurrence of another. The complete interference was confirmed by the cytogenetic observation that there was only one chiasma for all bivalents. Further, sites of the single crossover seem not to be randomly distributed along chromosomes, but preferentially located in a recombination hot-region proximal to the centromere. The restricted distribution of a single crossover per chromosome provides one explanation for the unique phenomenon of heterozygote deficiency in M. lateralis and other molluscs.