Herbicides are important in crop protection and management. A number of them including 2,4-D (2,4-dichlorophenoxyacetic acid), however, may reach water bodies and eventually affect the non-target organisms such as rotifers. In the present work, we studied the influence of 6 concentrations viz. 0 (control), 100, 200, 300, 400 and 500 mg l-1 of 2,4-D on the population growth of the rotifer Brachionus patulus under two algal (Chlorella) food levels (0.5 x 10(6) and 1.5 x 10(6) cells ml-1). Regardless of herbicide concentration, the population growth of B. patulus was dependent on the algal food levels, in that an increase in algal food level supported a better population growth. Similarly at any Chlorella density, the herbicide had a negative influence on the population growth of B. patulus. Herbicide level of 500 mg l-1 inhibited population growth of B. patulus beyond 5 days. Rotifers grown under low food density and high herbicide concentration (300 mg l-1 or above) were completely eliminated after day 15. The rate of population increase (r) (mean +/- standard error) in the controls varied from 0.46 +/- 0.002 and 0.55 +/- 0.004 under 0.5 x 10(6) and 1.5 x 10(6) cells ml-1 of Chlorella, respectively. The r values became negative under both, low and high food levels, at or beyond 300 mg l-1 of 2,4-D. The maximal population abundance (ind. ml-1) in controls varied from 294 +/- 9 to 503 +/- 21 under low and high food levels of Chlorella, respectively. The role of algae in mitigating adverse effects of high herbicide concentrations to rotifers has been discussed.