A new bioassay unit is described which provides simulated stream conditions for testing control agents against black fly larvae. Attributes include uniform flow, high control survival, and excellent visibility for observing larval behavior. It also possesses the unique advantage of using recycled water, continuously supplied fresh water, or both simultaneously. The unit has performed satisfactorily in assays against black flies using pathogens, parasites, and chemicals. In addition, stream invertebrates from four insect orders have been maintained in the unit.

A simple and inexpensive incubation unit is also described which possesses several of the advantages of the bioassay unit. It lacks the capability for recirculation and is generally less versatile than the bioassay unit; however, it is useful for the maintenance and observation of stream insects.