Fungal cell wall growth has been postulated to involve the cooperative action of both wall-hydrolyzing and wall-synthesizing enzymes (Park and Robinson, 1966; Gooday and Trinci, 1980). Investigations of cell wall morphogenesis in Achlya have dealt primarily with hydrolytic events, particularly those mediated by endo-l,4-D-glucanases (Thomas and Mullins, 1967, 1969; Hill and Mullins, 1979, 1980). Much less attention has been devoted to wall-synthesizing enzymes in Achlya and other cellulosic fungi because the activity of glucan-synthesizing enzymes is difficult to demonstrate in vitro, and the low level of activity makes characterization of the products uncertain. In a previous investigation, uridinediphosphoglucose (UDPG) transferase (EC 2.1.4.12) was demonstrated at low activity in association with subcellular particles that also exhibited cellulase activity (Hill and Mullins, 1980). Since these particles were similar to apical vesicles this association suggests that UDPG transferase in Achlya may be involved in cell wall synthesis. In the present paper, activity of UDPG transferase is demonstrated in cell walls of Achlya, and the solubility properties of the in vitro reaction products are described. Achlya ambisexualis Raper strain E87 was cultured for 48 h as described previously (Mullins, 1973). UDPG tranferase activity was assayed by a modification of the techniques of Ray et al. (1969) and Shore and Maclachlan (1975). The reaction mixture contained (mM): UDPG, 0.24; cellobiose, 5; MgCl2, 11; dithiothreitol (DTT), 1.7; pH 5.8 sodium phosphate buffer, 67; and incubation was 20 min at 23 C. The UDPG was glucose-uL-14C (250 utCi/Aumole, New England Nuclear) and as used gave 7.3 x 10-8 moles with 119 nCi of radioactivity. Products were recovered by centrifugation of 70% ethanol-insoluble material after addition of 30 mg of powdered Whatman cellulose as a carrier. To determine the distribution of transferase activity between wall and protoplasm fractions, mycelia were homogenized by ultrasonication at 5-10 C in a buffered homogenizing solution containing 20% w/w sucrose, 10 mM DTT, and 20 mM tris HCl buffer, pH 7.6. Hyphal fragments were sedimented by centrifugation, then resuspended, and the entire process repeated three times. The supernatant fractions were pooled and the final sediment, when examined by phase contrast microscopy, consisted of wall fragments without apparent cytoplasmic contamination. UDPG transferase activity was assayed in each fraction and protein was estimated with the Branford (1976) Bio-Rad assay. Results are displayed in TABLE I. Although less transferase activity was found in the wall fraction than 851