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Aflatoxins are secondary metabolites that are produced by toxigenic strains of some Aspergillus species on foods. Neem plant is a known inhibitor of aflatoxin production. We studied the effects of different concentrations of aqueous neem leaf extract on fungal growth and aflatoxin production by Aspergillus parasiticus (NRRL 2999) at different incubation times. The toxigenic fungus was cultured on sucrose low salts medium in the presence of various concentrations of extracts (0.2, 0.8, 3.12, 12.5 and 50% v/v). After shaking incubation of cultures for 2, 4, 6, 8, 10 and 12 days at 28 °C, the fungal mycelia was collected and processed for determination of dry weight. Mycelia and culture media were assayed by TLC method to detect aflatoxin B1 (AFB1). The extracts did not have any obvious effect on fungal growth. AFB1 production in the control samples increased to the maximum level on the 8th day. The inhibition of aflatoxin synthesis by plant extracts was found to be time- and dose-dependent. The maximum inhibitory effect was 80–90% in the presence of 50% concentration that when compared with control samples was significant (P < 0.05). AFB1 secretion/production ratio in all of control and treated samples, other than 2nd day, approximately stayed and neem had no effect on it.