The microbiological transformation rates of four organic compounds, the butoxy-ethylester of 2,4-dichlorophenoxyacetic acid (2,4-DBE), p-cresol, alpha-naphthol and quinoline, added to natural water samplers were examined in laboratory experiments. Graphical analysis of the data with first-order log plots indicated that transformation of these compounds occurred in two phases. The initial phase consisted of a lag period during which no decrease in compound concentration could be detected. Three of the compounds--p-cresol, alpha-naphthol and quinoline-- were only transformed following a lag phase. The transformation of 2,4-DBE occurred immediately upon addition of the compound to sample waters. The lag period was followed by a transformation phase where the detectable decrease in compound concentration could be described by a pseudo first-order rate equation and for which psuedo first-order constants could be determined. The variability in first-order constants for the different compounds ranged from a low of 13.6-fold for 2,4-DBE to a high of 185-fold for quinoline. Much of the variability could be accounted for in the range of average bacterial populations, measured during the transformation phase, that were used to calculate second-order rate constants and from the observation that second-order rate constants could be clustered into groups that were statistically different. The variability of second-order constants within these groups ranged from 1.18 to 36.14-fold, whereas the first-order constants ranged from 1.24 to 184.71-fold.