Drosophila ebony mutants exhibit a syndrome of morphological and behavioral phenotypes that include an abnormally dark body color and defects in visual and courtship responses. We now show that mutants carrying any one of five ebony alleles display complex and variable locomotor activity rhythms. Although in the most extreme cases activity is essentially aperiodic, many individuals express short- and/or long-period activity components. Three different ebony mutants (e, e1, and e11) express free-running rhythmicity in a temperature-dependent manner; activity rhythms are robust at 28 degrees C, but weak or absent at 20 degrees C. Even while maintained in a light-dark (LD) cycle, ebony homozygotes characteristically display extremely disorganized patterns of activity; some individuals entrain with an apparently abnormal phase and/or express multiple rhythmic components. Interestingly, the visual system mutation norpA partially suppresses effects of the e1 allele, which suggests that aberrant visual system inputs might contribute to the rhythm deficits of ebony mutants. In contrast to their effects on the locomotor activity rhythm, ebony mutations have no apparent impact on the circadian rhythm of adult eclosion, and thus exert rhythm-specific effects on circadian periodicity.