The estimate of the age of insects involved in corpse decomposition is important for the Post Mortem Interval (PMI) estimate, particularly when the PMI is based on information obtained from necrophagous insects. However, the information provided by these insects, particularly during their immature stages, may be distorted by the presence of toxic substances in the corpse, for they can affect the developmental time of the larvae feeding on the cadaver, resulting in an error of the PMI estimate. Phenobarbital is a widely used barbituric, having sedative, hypnotic, anesthetic and anti-epileptic effects. The aim of this study was to determine the effect of phenobarbital on the development of the immature stages of Chrysomya megacephala (Fabricius) (Diptera: Calliphoridae). The larvae were reared on an artificial diet containing the drug in 3 different concentrations: 1 g/kg, 500 mg/kg, and 150 mg/kg. The control group was reared on the diet containing no drug. The total mean developmental time up to the end of pupation was 180 h at the concentration of 1g/kg, significantly different from the 120 h of total development time at drug concentrations of 500 and 150 mg/kg, and the control group. The mean maximum larval weights were also affected by the drug, reaching 57.2±8.1 mg at the concentration of150 mg/kg after 144 h of development, and respectively 43.4±6.7 mg and 41.1±8.2 mg, both after 84 h of development, at concentrations of 150 mg/kg and 500 mg/kg.