The effects of thickening agents (polyethylene glycol [PEG] 20000 and carboxymethylcellulose [CMC]) on the penicillin fermentation by Penicillium chrysogenum were investigated. By adding the thickening agents to the fermentation medium, the growth form of the mold can be manipulated. Depending on the amount of thicking agent added, the change in morphology is from compact smooth pellets to various intermediate forms, and finally to filamentous mycelia. It was found that better penicillin production was obtained when the mold was in small, fluffy, loose pellets. The penicillin fermentation is not only affected by the thickening agents, but also the status of inoculum and agitation. Under the condition that the mold will otherwise grow in large pellets (e.g., under a low level of spore inoculum), the enhancement in the penicillin production through addition of the thickening agents may be more significant. In tank fermentation, the thickening agent was introduced in the stage of preculture, rather than main culture. The increase in the broth viscosity caused by addition of the thickening agent resulted in a decrease in dissolved oxygen level, which could be compensated in the case of PEG 20000.