A duplicated factorial approach was used to assess the combined effects of three environmentally realistic levels of alachlor (0.0, 5.0, and 90.0 μg/L), cross-classified with three environmentally realistic levels of atrazine (0.0, 12.0, and 150.0 μg/L), on indigenous epipelic algae (mud-dwelling species, characteristic of agricultural streams). The experiment was conducted in 18 recirculating laboratory streams over a 4-week period. Biovolume of viable algal cells and relative abundance of the six dominant algal species were utilized as experimental endpoints. Community biovolume was impacted throughout the experiment in streams treated with 12.0 and 150.0 μg/L atrazine and in streams containing 90.0 μg/L alachlor, but only in samples collected at 4 weeks. The effects of atrazine and alachlor together on algal community biovolume appeared to be additive rather than synergistic. The relative abundance of two dominant algal taxa were impacted in streams treated with alachlor, but only one species remained affected over time. The lack of significant interactions is most likely attributable to the different modes of action of atrazine and alachlor. The additive nature of these two herbicides should be an important consideration in the development of water quality criteria.