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1. The cyprid larvae of Balanus improvisus were found to settle in a diurnal rhythm with maximum numbers attaching during daylight hours.2. No consistent pattern of vertical distribution of the cyprids was found.3. The normal diurnal cycle in rate of attachment of barnacle cyprids was nullified by the use of artificial illumination over the collecting surfaces at night.4. The magnitude of the cyprid collection on the artificially illuminated surfaces was equal to the collection on the sun-illuminated surfaces in daylight.5. The intensity of artificial light necessary to produce large cyprid attachments at night was of an order as low as 1 footcandle at the water surface.6. No correlation was found between the quantity of artificial light at night and the numbers of cyprids attached.7. The highest rate of cyprid attachment relative to the phase of the tide was found to occur when the waters of upper Biscayne Bay were sampled at the collecting station. This body of water reached the sampling station at low ...