DNA SYNTHESIS in eukaryotes is asynchronous, with heterochromatin replicating later than euchromatin1. In Drosophila, autoradiographic studies of metaphase chromosomes from embryonic2 and cultured3 cells have shown that the centromeric heterochromatin of chromosome 2 is late replicating. The histone genes have been localised to subsections 39D and 39E in the left arm chromosome 2, close to the centromere4, and may be part of this late-replicating region for the following reasons. First, the five tandemly arranged histone coding sequences are reiterated approximately 50 times, and late-replicating DNA has been shown to contain significantly more repeated (but non-satellite) sequences than the genome as a whole5. Second, the histone genes are transcriptionally active in all but the final hours of the S phase, suggesting that their replication may be delayed6. Third, they are in close proximity to the centromere. Asynchrony has been detected in the recombination process as well. Heat treatment of Drosophila oocytes at sequential intervals during premeiotic-S phase elicits increases in recombination that follow a well-ordered pattern7. To investigate the possibility that the pattern of heat response parallels that of replication, the centromeric heterochromatin of chromosome 2 and the contiguous histone region have been examined for the time of their recombinational response to heat during the S phase. We present here evidence for late response in both regions. In addition, a level of response roughly two orders of magnitude greater than the average encountered in the genome has been found in these regions.