A temperature-sensitive (ts) third chromosome Minute (M) mutation, designated Q-III, has been recovered and characterized. Q-III heterozygotes raised at 29 degrees exhibit all of the dominant traits of M mutants including small bristles, rough eyes, prolonged development, reduced viability and interactions with several unrelated mutations. Q-III homozygotes raised at 29 degrees are lethal; death occurs primarily during the first larval instar. When raised at 22 degrees, Q-III heterozygotes are phenotypically normal and Q-III homozygotes display moderate M traits. In addition, Q-III elicits ts sterility and maternal-effect lethality. As is true of M lesions, the dominant traits of Q-III are not expressed in triploid females raised at 29 degrees. Complementation tests suggest that Q-III is a ts allele of M(3)LS4, which is located in 3L near the centromere.--Reciprocal temperature-shift experiments revealed that the temperature-sensitive period (TSP) of Q-III lethality is polyphasic, extending from the first instar to the latter half of pupation. Heat-pulse experiments further resolved this into two post-embryonic TSPs: one occurring during the latter half of the second larval instar, and the other extending from the larval/pupal boundary to the second half of pupation. In addition, heat pulses elicited a large number of striking adult phenotypes in Q-III individuals. These included pattern alterations such as deficiencies and duplications and other morphological defects in structures produced by the eye-antennal, leg, wing and genital imaginal discs and the abdominal histoblasts. Each defect or pattern alteration is associated with a specific TSP during development.--We favor the interpretation that most of the major Q-III defects, particularly the structural duplications and deficiencies, result from temperature-induced cell death in mitotically active imaginal anlagen, while the small macrochaete phene probably results from the direct effects of Q-III on bristle synthesis. The hypothesis that the Q-III locus specifies a component required for protein synthesis is discussed, and it is concluded that this hypothesis can account for the pleiotropy of Q-III, and that perhaps it can be extended to M loci in general.