Values of LC50, LC90 and LC99,9 of phosphine generated from PHOSTOXINRpellets for third instar larvae and pupae of Sitotroga cerealella at 20 and 28°C for fixed exposure periods of 2, 4, 8, 24, 48 and 72 h were determined and presented. It was found that the values of the lethal concentrations of phosphine considerably decreased with increasing exposure periods. The gas was less effective at short exposure periods (2—8 h) than at long exposure ones (24—72 h). The CT-product required at a given exposure time for 50 or 90 % kill of the insect stages was greater at 20 than at 28°C. Pupae of S. cerealella were more tolerant to phosphine than larvae. Various carbon dioxide concentrations (20, 50, 78 %) applied alone showed a negligible effect on larval and a slight effect on pupal mortalities at short exposure periods, but the mortalities resulting at longer exposure times (24-72 h) were found between 4—60 % at the two test temperatures. Larval mortalities obtained from mixtures of the LC50 of PH3 + CO2 were significantly higher than those of each gas alone, whereby a synergistic action was proved for all exposure periods at 20°C and 28°C. An increase in pupal mortalities was achieved at all exposure periods with exception of 8 h at 20°C, and an independent joint action was found at short and long exposures at 28°C.