The mucoraceous fungi, Mucor hiemalis and Zygorhynchus heterogamous, are considerably more susceptible to paraquat than the imperfect species, Aspergillus niger and Penicillium frequentans. The inhibitory effect of increasing levels of paraquat on hyphal extension, dry matter production and spore germination are much more marked in Mucor and Zygorhynchus. When grown on media containing paraquat, the mycelia of these species contain approximately twice as much of the herbicide as those of Aspergillus or Penicillium. Feeding (14C-methyl)-paraquat confirms that twice as much activity is accumulated by the susceptible species. It is suggested that susceptibility to paraquat is associated with the degree to which the herbicide accumulates in the mycelium.