Two single locus Variable Number Tandem Repeat (VNTR) DNA probes were used to test for differences in allele distribution between precociously mature male and immature chinook salmon, Oncorhynchus tshawytscha. Two populations were examined: Robertson Creek (RC) adult salmon, and Nicola River (NR) freshwater juveniles, or parr. Genomic DNA was extracted from 74 RC precociously mature adult males ('jacks') and 94 RC immature adults of the same age and from 45 NR precociously mature parr and 51 NR nonmaturing parr. The genomic DNA was hybridized with a single locus VNTR probe developed for chinook salmon (OtSL1), as well as one developed for Atlantic salmon, Salmo salar (Ssa1). The allele frequency distributions at both loci were significantly different for the RC jacks and immature fish, indicating a family effect on the incidence of precocious maturation in that population. No difference was found between the allele frequency distribution of the NR precocious and immature parr. A bin width sensitivity analysis showed that the comparisons of the allele frequency distributions were insensitive to the choice of bin size. No differences in heterozygosity were found between mature and immature fish at either locus for both stocks. Preliminary testing for family effects on phenotypes of interest, such as alternative life history strategies, can be performed using hypervariable VNTR DNA probes, prior to implementing costly and involved breeding programmes.