Inheritance and genetic linkage of 29 allozyme loci were studied by single-pair crosses of Yponomeuta padellus (Lepidoptera, Yponomeutidae). All loci segregated as Mendelian genes with codominant alleles except for a null allele at the hbdh locus. The three loci est-2, 6pgdh, and fudlt were sex-linked and occurred in that order along the Z chromosome. Autosomal linkage analysis was facilitated by the lack of crossing-over in females characteristic of Lepidoptera, because linkage in female-informative crosses is all-or-none and observation of 'forbidden recombinants' provides conclusive evidence that two loci are not syntenic. Convincing evidence was found for linkage of the autosomal loci me and mpi. This linkage group and aat-1, ca, hbdh, hk-1, pgi and pgm were assigned to seven separate autosomes. Weaker evidence from male-informative families supported four additional linked pairs. Previous cytological studies have shown that the sex chromosomes in heterogametic females are associated in a trivalent, consisting of a W chromosome translocated to an autosome (A(w)), paired with the Z chromosome and the homologous autosome A. Any locus located on this autosome and its homologous segment on the A(w) chromosome should show strict segregation of one allele to sons and the other to daughters. However, none of the female-informative loci in this study satisfied that criterion. Use of the method of forbidden recombinants and its utility in the study of chromosomal evolution in Yponomeuta are discussed.