Highly repetitive DNA of C0t 0--0.2 was purified from whole DNA of Triturus cristatus carnifex, labelled by nick translation, and in situ hybridized to RNA transcripts on the loops of lampbrush chromosomes and to the DNA of mitotic chromosomes from intestinal epithelium from the same species. The labelled DNA bound to 20--30 loops on the long arms of lampbrush bivalent 1, a pair of loops near the centromere on bivalent 10, and a number of other loops most of which were localized in pericentric regions. In mitotic preparations the same labelled DNA bound to the heteromorphic regions of the long arms of both chromosomes 1, and to the centromeric regions of all chromosomes. Centromeric labelling was light on chromosomes 4 and particularly clear on the 3 shortest chromosomes of the set. The heavy labelling of the heteromorphic arms of chromosome 1 is discussed in relation to several other peculiar properties of these arms, including their extraordinary lengths, their Giemsa banding patterns, and the absence of meiotic crossing over. It is suggested that insofar as the results with DNA/DNA hybridization and mitotic chromosomes match those obtained with the DNA/RNA-transcript hybridization and lampbrush chromosomes, confidence in the latter technique may be increased accordingly.