The diel emergence periodicity of Chironomids in a shallow residential-recreational lake in southern California was investigated on 4 occasions between May and October 1977. Emerging adults were sampled as 2-hr intervals during a 24-hr period with submerged emergence traps. Diel periodicity of emergence was exhibited by 6 species of Chironomids. Emergence of Tanytarsus (2 species) continuing throughout each diel period, accelerated considerably 2-3 hr before dusk with most emerging within 2-3 hr after sunset. Emergence activity of Chironomus decorus Johannsen, and Chironomus frommeri Atchley and Martin, was essentially nocturnal between dusk and midnight and having a pronounced peak within 2-3 hr after sunset. All adults of Dicrotendipes californicus (Johannsen) emerged between dusk and 1-2 hr after sunset. Adults of Procladius freemani Sublette showed a district bimodal eclosion pattern with peaks occurring during the crepuscular periods. The diel emergence periodicity of all 6 Chironomid species correlated with the diel light changes but fluctuations in water temperature exerted no observable effect on their emergence activity.