In order to determine the larvicidal action of different insecticides against both susceptible and resistant blowfly strains in the Republic of South Africa, an attempt was made to find a suitable in vitro method for mortality counts and potency evaluations. After a series of preliminary trials the Australian method of Roxburgh & Shanahan (1973) was adopted as the most reliable for this purpose. A statistical analysis by computer of the basic data at Onderstepoort established the superiority of the LC50 value in biological assays over the LC99 or the closely related minimum lethal concentration (MLC). The insecticidal action of different blowfly compounds was evaluated for relative potency against both susceptible and resistant blowfly strains by comparing their LC50 values in their logarithmic form. Comparisons are fully justifiable where the individual regression coefficients of the insecticides under test do not deviate significantly from a common slope initially calculated (Finney, 1971). A diazinon formulation proved the most potent against susceptible blowflies and fenthion ethyl against the resistant strain from Riversdale. Relative insecticidal potencies varied from 0, 07 - 0, 62 in the former compound and from 0, 35 - 0, 66 in the latter.