Sequence-tagged-site (STS) markers of arbitrary genes were investigated in black spruce [Picea mariana (Mill.) B.S.P.]. Thirty-nine pairs of PCR primers were used to screen diverse panels of haploid and diploid DNAs for variation that could be detected by standard agarose gel electrophoresis without further manipulation of amplification products. Codominant length polymorphisms were revealed at 15 loci. Three of these loci also had null amplification alleles as did 3 other loci that had no apparent product-length variation. Dominant length polymorphisms were observed at 2 other loci. Alleles of codominant markers differed in size by as little as 1 bp to as much as an estimated 175 bp with nearly all insertions/deletions found in noncoding regions. Polymorphisms at 3 loci involved large (33 bp to at least 114 bp) direct repeats and similar repeats were found in 7 of 51 cDNAs sequenced. Allelic segregation was in accordance with Mendelian inheritance and linkage was detected for 5 of 63 pairwise combinations of loci tested. Codominant STS markers of 12 loci revealed an average heterozygosity of 0.26 and an average of 2.8 alleles in a range-wide sample of 22 trees.