Behavior of a series of fusion proteins of varying charge in reversed micellar extraction was studied. The proteins consisted of the enzyme glucoamylase from Aspergillus awamori joined to short peptides containing from 0-10 additional aspartate residues. The fusions were partitioned into two different cationic surfactant systems, one based on the surfactant trioctylmethylammonium chloride (TOMAC) and the other on cetyltrimethylammonium bromide (CTAB). These two systems differed chiefly in micelle size as measured by the water to surfactant ratio, wo. Water numbers were determined for the TOMAC system, with values of approximately 10, and as a function of pH and ionic strength for CTAB for each of the mutant enzymes. For the CTAB system, water numbers were as low as 50 with NaCl concentrations of 500 mM and as high as 68 at 300 mM NaCl (95% confidence level of 2.4). The enzyme partitioned most strongly using CTAB, with maximal recoveries approaching 95%. However, in the CTAB system, there were no significant differences in behavior between the mutants because of the relatively large micellar size, even under high salt concentrations. Extraction of the control enzyme from clarified cell broth indicated that broth components did not significantly interfere with partitioning.