The kinetics of the reaction catalyzed by acyl-CoA: sn-glycerol-3-phosphate O-acyltransferase solubilized from Euglena gracilis microsomes were examined. For myristoyl-, palmitoyl-, stearoyl-, and oleoyl-CoAs, the initial reaction rates rose with increasing substrate concentration up to an optimal concentration that varied from 18.5 to 25 μ M, well above the respective critical micelle concentrations. At higher substrate concentrations, reaction was progressively inhibited. Arachidoyl-CoA was a relatively poor substrate for the acyltransferase, and substrate inhibition was not seen with it. Km values for acyl-CoAs ranged from 13 to 20 μ M while the corresponding V values varied almost 40-fold. Bovine serum albumin, among other effects, caused a change in the kinetic pattern of the reaction acyl-CoA dependency. Both acyl-CoA micelles and albumin-bound acyl-CoA were substrates. The binding of palmitoyl- and oleoyl-CoA was 2.7 and 1.5 mol, respectively, per mol of albumin. The critical micelle concentration of palmitoyl-CoA under the reaction conditions was shown by low angle light scattering photometry to be 7.1 p.M. The sn-glycerol 3-phosphate concentration dependency of the acyltransferase initial velocity exhibited Michaelis-Menten kinetics with Km values of 1.3 and 2.9 mM in the presence of 12.5 and 25 μM palmitoyl-CoA, respectively. The substrate analogues sn-glyceraldehyde 3-phosphate and dihydroxyacetone phosphate inhibited the reaction.