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Dursban, an organophosphorus insecticide, was applied on June 3, June 18, and July 1 to eight shallow (24 cm) experimental ponds near Bakersfield, California. Four ponds were treated at 0.028 kg/ha (=0.025 lb./acre), four were treated at 0.28 kg/ha, and four were kept as controls. Phytoplankton, zooplankton, and insect populations were sampled on 21 dates between May 20 and August 16. Insect larvae and nymphs were more reduced in numbers than were insect adults, reflecting in part the ability of the latter to emigrate and immigrate. Twenty—four—hour posttreatment samples for the second and third, but not first, treatments showed greater reductions of predaceous Notonectidae, Dytiscidae, Coenagrionidae, larval Hydrophilidae) than of "herbivorous" (Corixidae, Baetidae, adult Hydrophilidae) insect populations. Predaceous insect populations generally recovered to control—pond levels more slowly than herbivorous insect populations. Five weeks after the last insecticide treatment, predaceous insects averaged only 45% and 9% as abundant in low dose and high dose ponds, respectively, as they were in control ponds (P < 0.02); corresponding figures for herbivorous insects were 108% and 206% (P < 0.10). Initially, Cyclops vernalis and Moina micrura were the only crustacean zooplankters present, and both experienced high mortality due to treatments. Recovery in low dose ponds was variable, requiring 1—3 weeks, and in high dose ponds occurred only 3—6 weeks after the final treatment. Populations of Diaptomus pallidus seemed unaffected by the lower rate of treatment, but became large only after Cyclops populations had been reduced by the insecticide or other factors. Ceriodaphnia sp. became abundant in late July or early August in all four control ponds but not in any treated ponds. Herbivorous rotifers, principally in the genera Brachhionus, Polyarthra, Hexarthra, Filinia, and Tripleuchlanis, increased dramatically, often five— to twentyfold, within 1—3 days after Moina and Cyclops populations were decimated by Dursban, but became abundant in control ponds only during the brief absence of Moina in early July. All planktonic rotifers, except Polyarthra, were more abundant in treated than in contro ponds; all benthic—littoral rotifers (e.g., Lecane, Monostyle, Tripleuchlanis, Platyias, Lepadella, Testudinella) were more abundant in control than in treated ponds. The predaceous rotifer Asplanchna brightwelli was 35 times more abundant in treated than in control ponds, feeding primarily on herbivorous rotifers, sometimes on Cyclops, Diaptomus, Moina, and large algae, and rarely on small mayfly (Baetidae) nymphs and chironomid larvae. Asplanchna populations were composed of three morphotypes, ampulliform, cruciform, and campanuliform, the last of which tended to be highly cannibalistic. The "wings" of humps of the cruciform morphotype apparently represent a mechanism for minimizing such cannibalism. The reduction of herbivorous crustaceans by Dursban and the restraint of herbivorous rotifers by Asplanchna predation permitted the rapid increase of phytoplankton populations in treated ponds and, even 6 weeks after the last treatment, the phytoplankton was two and 16 times more abundant in low dose and high dose ponds, respectively, than in control ponds. Blooms of bluegreen algae (Anabaena, Anabaenopsis) developed in three high dose and one low dose pond, and a bloom of the diatom Synedra developed in one low dose pond. The long—spined alga Schroederia setigera seemed favored by high Moina and Diaptomuspopulations and low Cyclops populations. The significance of these results for insect—control programs is discussed.