The uptake of phenoxyacetic acid by two different strains of Penicillium chrysogenum was studied. Phenoxyacetic acid (POA) was taken up by P. chrysogenum in a defined medium. Plots of initial velocity of POA uptake versus external substrate concentration, in the range 2-5000 mu M, gave linear plots. Uptake of POA by induced and uninduced cells was identical. The initial velocity of POA uptake decreased as the pH of the suspension was increased : from 5.4 to 7.2; the decrease closely paralleled the decline in the non-ionic form of the acid over this pH range. The initial velocity of POA uptake was not affected by the presence of phenylacetic acid. POA uptake proceeded until the cellular concentration was equal to the external concentration, It is concluded that POA is passively transported into P. chrysogenum by unmediated diffusion.