Microencapsulated permethrin (penncapthrin) was evaluated under laboratory conditions for its toxicity toward several nontarget aquatic invertebrates. Average LC50 estimates for selected lotic invertebrates, based on a one hour dosing regime, were: 2.71 mg/L for Simulium vittatum, 4.59 mg/L for Hydropsyche spp., and 13.41 mg/L for Isonychia bicolor. In acute static tests with Daphnia magna, there was no significant difference (p less than or equal to 0.05) between the toxicity of penncapthrin at 96 h (LC50 range: 6.80-22.5 micrograms/L) and the EC formulation at 72 h (LC50 range: 0.6-21 micrograms/L). Comparatively, the toxicity of microencapsulated methyl parathion (penncap-m) was not significantly different from that of penncapthrin toward D. magna, the former having LC50 estimates ranging form 0.3-12.25 micrograms/L. LC50 estimates associated with Daphnia pulex ranged from 19 to 131 micrograms/L. The toxicity of penncapthrin and penncap-m toward D. pulex was difficult to determine because of frequent control mortality due to food deprivation resulting from the need to run tests for longer than 48 h. In successful tests, LC50 estimates ranged from 19 to 28 micrograms/L for penncapthrin and 0.08 to 25 micrograms/L for penncap-m after 72 h exposure. In long term toxicity tests, 95% of D. magna at 1 microgram/L, 44% at 10 micrograms/L, and 20% at 15 micrograms/L survived after 39 days exposure. Less than 15% of D. pulex survived over the same concentration range following 32 days exposure. Despite same drawbacks, long-term toxicity tests were more appropriate than short-term tests for evaluating microencapsulated pesticides because of reduced variability in LC50 estimates and lower control mortality.(ABSTRACT TRUNCATED AT 250 WORDS)