In the secondary constriction region of the short arm of human group D and group G acrocentric chromosomes there are tandemly repeated sequences of transcriptable segments and non-transcriptable spacers of rRNA genes. In the non-transcriptable spacer segments there is a polymorphic region with length variation. This paper reports the results obtained when the BamHI digested products of genomic DNA from 36 cases of Han nationality and those from 40 cases of Yi nationality were probed with a cDNA subprobe, pA(BE), which corresponds to the downstream one third of the 28S rRNA gene and is 1.6kb long with BamHI and EcoRI restriction sites at its two terminal respectively, using Southern blotting and molecular hybridization techniques. A total of 10 different restriction fragments were detected. Four fragments (7.9, 4.9, 4.0 and 1.2 kb) were seen in all subjects tested and the frequencies of six other polymorphic fragments in Hans and Yis (10.0, 9.4, 8.5, 6.7 6.0 and 3.0 kb) showed no significant difference when compared to each other. From the 10 different patterns of combination of restriction fragments four of them (1HY, 2HY, 3HY and 4HY) were detected in both Hans and Yis with no significant difference between their respective frequencies. Four other patterns (5H, 6H, 7H and 8H) were only seen in Hans with individual frequencies not high enough to be statistically significant. However, the statistical test for the combined frequencies of these four patterns showed highly significant difference between Hans and Yis (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)