The aim of this study was to determine the median effective concentration (EC50) of carbofuran, a carbamate pesticide, for Daphnia magna and to evaluate the effects on the heart rate. In the experiment, five neonates (≤ 24 hours) were used for each negative control, solvent control (acetone) and the treatment groups. The treatment groups were exposed to five different concentrations (2.5 μg/L, 4.5 μg/L, 8.1 μg/L, 14.58 μg/L and 26.244 μg/L) of carbofuran (98%) for 48 hours. Immobilised daphnids were noted at the 24th and the 48th hour in both control and treatment groups. EC50 values were calculated by probit analysis as 33.814 μg/L and 3.451 μg/L for 24 and 48 h, respectively. At the end of the 48th h, all samples were observed by light micros- copy and the heart beats per minute (bpm) were counted by using a video camera. Heart rate was similar in negative and solvent control groups (444.8±24.12 and 458.4±34.14, respectively). In the treatment groups heart beats were noted as 498.4±17.43, 183.8±103.83, 441.6±35.73, 293.8±101.09 and 442.4±32.94 for 2.5, 4.5, 8.1, 14.58 and 26.244 μg/L, respectively. These results show that carbofuran caused immobilisation and decreased heart rate in D. magna. However, heart occured independently of carbofuran concentration gradient.