Changes with age in the vertical migratory behavior of Chaoborus flavicans larvae were studied in a small, eutrophic lake by use of Clarke‐Bumpus plankton samplers and a high frequency echo sounder. Marked differences were noted in vertical distribution and diel migration of the four instars. Diel and seasonal changes in timing of ascent and descent of larvae appeared to be controlled by subsurface illumination. Results from observations of vertical migration of different instars held in experimental tubes housed in a darkroom corroborated those of the field. Complete, three‐phase migration cycles were induced by artificially changing natural light intensity over the experimental tubes during midday. These and other experiments suggested that the diel vertical migration of the larvae was an exogenous rhythm controlled by light.