Segments of repeating DNA, termed variable number tandem repeats, within the genome are often used as polymorphic markers. Here we report such a repeat in the intervening sequence 25 of the type III procollagen gene (COL3A1) located at 2q31-q32.3. The region containing intervening sequence 25 of the COL3A1 gene was amplified using the polymerase chain reaction and the products were analyzed by agarose gel electrophoresis and dideoxynucleotide sequencing. The repeat consisted of an AT-rich unit of 15 base-pairs (ATATATATATGAGAC). Seven different alleles were identified containing between three and nine repeats of the 15 base-pair unit. The allele heterozygosity was 73% in 117 unrelated individuals. The alleles were shown to be inherited in a Mendelian fashion in three large families. This marker should prove useful in linkage studies for COL3A1, and for linkage studies of other genes on the long arm of chromosome 2. The information extends and complements a report on the same repeat recently published by Lee et al. (J. Biol. Chem. 266: 5256, 1991 a).