The basic characteristics of the endogenous diurnal rhythm of pupation in the black salt-marsh mosquito, A. taeniorhynchus (Wiedemann), have been investigated in detail. A distinct sinusoidal pattern of pupation, which is evident only under continuous darkness, was the result of simultaneous hatching of the eggs. A nonrepeated light pulse synchronized each peak of pupation and established a new phase for the rhythm. If the nonrepeated light pulse was at least 4 hours in duration, it was effective in phase-establishment as early as immediately after simultaneous hatching of the eggs and as late as 72 hours after hatch. The amount of phase-shift was proportional to the duration of the light stimulus, the greater phase-shift resulting from the longest signals. A light pulse, given at different phases of the subjective light-dark (LD) cycle, showed that sensitivity to light stimuli was greater during the first 12 hours than during the second 12 hours of the subjective LD cycle, and the phase-response curve obtained for the pupation rhythm under standardized conditions was very similar to those reported for different single animals and populations. The persistence of the pupation rhythm under constant conditions of light and temperature following alternate conditions, the distinct displaying of phase-shift, transients, and phase resetting substantiate that this pupation rhythm is an endogenous diurnal rhythm. LD cycles of less than 24 hrs, varying from 12 to 22 hours, imposed during earlier stages of development did entrain the pupation rhythm, although these LD cycles were not learned. Phase-shifts, transients, and phase-resetting by temperature perturbations differ from those generated by light perturbations. The temperature stimuli produce spectacular transients with relatively little phase-shift in the ultimate steady-state, whereas light pulses have spectacular effects on the steady state phase-shift. It is concluded that though the diurnal rhythm of pupation in A. taeniorhynchus is endogenous in its characteristics, it is suitable to study neither time measurement nor the general properties of circadian clocks. However, this endogenous rhythm is valuable in studying the physio-ecological processes in this insect.