Drosophila melanogaster possessing a temperature-sensitive (ts) mutation that maps to an X-linked locus ( RpII215 ) (the locus has also been called l(1)L5 and Ultrabithorax -like or Ubl ) encoding a subunit of RNA polymerase II are fertile at 22 degrees C but become sterile when shifted to 29 degrees C. Homozygous RpII215ts adult females shifted to 29 degrees C lay structurally normal eggs for 24 hr, after which increasing numbers of eggs are abnormal. Eggs left to develop at 29 degrees C die as morphologically normal late embryos or first instar larvae when produced by females maintained at 29 degrees C for less than 6 hr. However, eggs produced by females undergoing oogenesis at 29 degrees C for longer than 6 hr develop abnormally, displaying holes primarily in their ventral cuticle and possessing an abnormal pharyngeal apparatus. As exposure of females to 29 degrees C lengthens there is an increase in the severity of these defects. Some of the eggs can be rescued by either mating RpII215ts females to wild-type males or shifting the eggs to 22 degrees C. The percentage of eggs rescued decreases with increased length of oogenesis at 29 degrees C, up to 20 hr, at which point they are no longer rescuable. The terminal phenotype of eggs that fail to be rescued by the above procedure is less extreme than that of eggs for which no rescue attempt was made. Holes in the ventral cuticle are reduced or absent, but pattern formation is disrupted such that segments are often missing, incorrectly oriented or fused. Because the RpII215 locus encodes a subunit of RNA polymerase II, the developmental defects described above are most likely due to reduced or aberrant transcription during oogenesis and early embryogenesis. This postulated effect on transcription results, in part, from the maternal loading of a gene product(s) that is thermolabile in eggs.