A steady-state gradient apparatus was used to study population dynamics and to investigate chemical, physical, and behavioral characteristics influencing populations. Preferential localization of the cladoceranMoina brachiata (Jurine) was observed within the experimental gradient. Laboratory and field studies suggest thatM. brachiata responds to light as a discriminated stimulus when light is paired with a reinforcer, food. The cladocerans preferred as a medium water having a five-day biological oxygen demand (BOD) of approximately 14–16 ppm. Preference for BOD levels appears to be more restrictive than preference for levels of dissolved oxygen. The relationship of space and time with respect to ponds and gradient is discussed.