Circadian rhythms have been observed in individuals of all major taxonomic categories except bacteria and blue-green algae, yet little is known about the cellular or molecular nature of the oscillations underlying the overt rhythmicity. This project will utilize a biochemical genetic approach in which single-gene mutations affecting the circadian clock will be used to identify specific biochemical functions required for normal clock operation. The organism to be used is Neurospora crassa, for which a vast backlog of genetic and biochemical techniques and information is available. This fungus expresses rhythms of conidiation (asexual spore formation), hyphal branching, and CO2 output. Several new mutants which alter the period length of the first of these rhythms have been isolated and new techniques have been worked out to increase the efficiency of mutant screening procedure. Recent findings of biochemical oscillations in many areas of intermediary metabolism emphasize the importance of oscillations as an inherent property of biological control systems, while the ubiquity of circadian oscillations, encompassing micro-organisms, higher plants, and higher animals, clearly points out their importance in biological organization.