Spontaneous sex-linked recessive lethal mutation frequencies may vary with time. Examination of approximately 1.7 X 10(6) Drosophila melanogaster X chromosomes in three laboratories suggests two sources of variation in the spontaneous mutation frequency. First, the underlying mutation rate may change with time, suggesting genetic drift among the genes controlling the spontaneous mutation rate. In one set of experiments, the recessive lethal frequency increased from 0.2% to 0.4% in 6 years. Second, there may be seasonal variation in the mutation frequency even under apparently controlled conditions, suggesting that uncontrolled environmental factors may play an important role in determining the mutation frequency. Therefore, concurrent controls should be used in mutation experiments except in cases in which the frequency in the untreated control is so low as to be unmeasurable in a single experiment or the control frequency can be shown not to vary with time.