AbstractA. niger, growing in the presence ofp‐aminobenzoic acid and its lower alkyl esters, departs significantly from linearity in media of approximatepH3 to 5, containing nitrate nitrogen. This effect is not observed where media are ofpH6 to 7 or nitrogen is supplied solely as the ammonium ion or the freepara‐amino grouping is acetylated or with the correspondingottho‐ or meta‐aminobenzoate. The non‐linear growth rates can be interpreted in terms of an initial phase, a phase of secondary inhibition and a period of outgrowth. Inhibition in the initial phase is appreciably less at higherpHlevels.P. roquefortigrowing in the presence of esters ofp‐aminobenzoic acid in media wherepHrises during growth shows an accelerating curve. This effect is associated with a marked loss of inhibitory power atpHgreater than about 5·7. A similar acceleration is seen with methyl N‐acctyl‐p‐aminobenzeate but has not been found with methylp‐hydroxybenzoate. None of the free aminobenzoic acids departs from the linear growth curve with this organism.B. fulvagives an acceleration in the presence of methylp‐aminobenzoate under the same conditions ofpHand nitrate nitrogen as cause secondary inhibition ofA. niger.In contrast to the other two organisms the less acid media (pH5 and 7) are relatively more inhibitory, this being associated with absence of acceleration such as is seen under the more acid conditions. The three aminobenzoic acids giv