Pelleting of hyphae of Rhizopus arrhizus commences 9-10 h after inoculation, and not at a pregermination stage as for other fungi. The anionic polymers carboxymethylcellulose (CMC) and Carbopol-934 prevent pelleting, causing dispersal of growth of most species of Rhizopus, including R. arrhizus, when used as media supplements. The non-ionic polymer methylcellulose also dispersed growth, but less effectively. The specific growth rate of mycelia in control (unsupplemented) and in polymer-supplemented media was similar until 24 h, when growth became restricted in control medium. Polymer supplementation promoted increased biomass production, but in the case of Carbopol, this increase may be partly due to the tightly bound polymer, as indicated by scanning electron microscopy and biomass studies, particularly at low pH's. Hyphal extension zones and branch points of mycelia from Carbopol-supplemented media at low pH did not fluoresce when treated with Calcofluor White, unlike those from control, CMC- or Carbopol-supplemented media at higher pH values. The cell walls of dispersed mycelia from viscous media had a higher concentration of hexosamines (34-35%) and a lower concentration of protein (7-6%) compared to the cell walls of pelleted (24% hexosamines/18% protein) or less finely dispersed mycelia (23% hexosamines/13% protein). Results from acid/alkali extraction indicate that a greater proportion of glucuronan is held in non-glucosamine linkages in walls from pelleted than from dispersed mycelia. The analysis of the chitin component of the cell walls by X-ray diffraction demonstrated less crystallinity in the cell walls isolated from Carbopol-supplemented media compared to the other cell wall types examined. The presence of solid or semi-solid medium components were necessary to promote production of glucoamylase by R. arrhizus. The presence of Carbopol inhibited glucoamylase production. It was also shown to inhibit fumaric acid production when mycelia were grown in' its presence and when used in the biotransformation media using pre-grown mycelia. Dispersed filamentous mycelia from CMC-supplemented medium produced the highest yields of fumaric acid in batch culture and in biotransformation studies. However in pH controlled fermenters, where higher aeration/agitation rates prevented densley-packed pelleting, the control mycelia produced the highest yields. Dispersed mycelia from CMC-supplemented medium also had the highest specific capacity to convert progesterone to 1la-hydroxyprogesterone. 48 h congealed mycelia from control medium manifested a low metabolic rate and poor rates of biotransformation.