The importance of gas phase diffusion in insect gas exchange remains unclear. The role of diffusion in gas exchange of developing Hyalophora cecropia pupae was examined by altering the gaseous diffusion coefficient in the breathing mixture. Gaseous diffusion coefficients were manipulated by substituting helium or sulfur hexafluoride for the nitrogen usually present in air. Sensitive mass loss recordings were employed to monitor gas exchange activity. Mass loss recordings showed a two‐phase cycle, open and closed‐flutter. Mass loss rates during the open and closed‐flutter periods were not altered in proportion to the changes induced in the rate of diffusion. Open‐phase duration was inversely and proportionally related to the diffusion coefficient. These results are consistent with changes in spiracle resistance or convective flow during the open period in response to a change in the diffusion coefficient. In addition, they indicate a significant gas phase diffusive resistance within the pupal tracheal system. This previously unreported gas phase resistance appears to be a major determinant of the duration of the open period and thus of overall water loss rates in these pupae.