to determine the period of harvest that optimizes the antimicrobial activities of the essential oil of ocimum gratissimum l. from benin, aerial plant parts were collected at two vegetative stages (pre- and full-flowering) and three sampling times (7 am, 1 pm, and 7 pm). extraction by hydrodistillation yielded between 0.65 and 0.78% of essential oils. characterization of the oils by gc-fid and gc/ms analysis revealed the presence of monoterpenes (87.26-93.81%), sesquiterpenes (5.57-11.34%), and aliphatic compounds (0.15-0.18%), with p-cymene (1; 28.08-53.82%), thymol (2; 3.32-29.13%), -terpinene (3; 1.11-10.91%), -thujene (4; 3.37-10.77%), and -myrcene (5; 4.24-8.28%) as major components. two chemotypes were observed, i.e., a p-cymene/thymol and a p-cymene chemotype, for plants harvested at 7 am for the former and at 1 pm or 7 pm for the latter, respectively. the oils were fungicidal against candida albicans, with the sample from full-flowering plants collected at 7 am being the most active (mic = 0.06+/-0.00 mg/ml). the chemical variation of the oils also influenced the antimicrobial effect against staphylococcus aureus; the most active oil was obtained from plants at the pre-flowering stage collected at 7 am (mic=0.24+/-0.01 mg/ml). escherichia coli was insensitive to the chemical variation of the oils (mics of ca. 0.48+/-0.02 mg/ml for all oils). moreover, the essential oils showed low toxicity against artemia salina leach larvae, with lc(50) values in the range of 43-146 g/ml. this is the first study of the interaction between the daytime of collection and vegetative stage of the plants and the antimicrobial properties and toxicity of the essential oil of o. gratissimum from benin.