A response surface methodology (RSM) was used to evaluate how pH and ionic strength (IS) affect the fate (i.e. size and colloidal stability) of an SC formulation containing the pyrethroid β-cyfluthrin. The response surfaces determined under a range of environmentally relevant conditions were then used to assess the toxicity of the SC formulation of β-cyfluthrin to D. magna. The changes in hydrodynamic diameter (HDD) and colloidal stability as determined by zeta potential measurement were closely related to either or both of the change in pH and IS with the linear factor of IS being the most significant factor affecting those changes. Thus, the concentration of SC formulation of β-cyfluthrin remaining in the water column was dependent on the pH and IS conditions and highest when the colloidal suspension contained small particles or a lack of agglomeration leading to sedimentation of the particles. The toxicity results show correspondingly higher toxicity to D. magna when exposed to the SC formulation of β-cyfluthrin when pH and IS conditions favor formation of either the smallest HDD or most stable colloidal suspensions.