There have been several recent studies to determine the controlling mechanism for the growth of microorganisms on hydrocarbons. 1 , n , 7 -9 While experimental results are in broad agreement, the proposed mechanisms are often in contradiction. A recent paper by Mallee and Blanch' is an excellent review of the suggested mechanisms. They also propose a new model for the transport of hydrocarbon based on these mechanisms. The basic aspect of their model is in agreement with the micelle model;? that the actual form of hydrocarbon, when it is assimilated by the microorganism, is that of a submicron sized particle. The cells elaborate lipophilic materials that act upon the hydrocarbon substrate to produce micelles; the large hydrocarbon droplets essentially serving only as hydrocarbon reservoirs.'While flocs play an important role in hydrocarbon transport to cells, Mallee and Blanch's assumption that hydrocarbon uptake is restricted only to cells that are part of a floc is questionable. A floc, by Mallee and Blanch's definition, may range from one cell and one substrate droplet to an entity containing many cells, substrate droplets, and air bubbles. Hence it excludes cell-and hydrocarbon-containing micelle combinations, which may or may not be a part of the floc.Owing to agitation, the flocs are continually being broken and reformed, during which many of the single cells may not become a part of the floc, but instead, freely exist outside the floc. Similarly, the extracellular products responsible for the micelle formation are present in the medium outside the flocs. These free cells will interact with submicron micelles formed by extracellular products in the broth. Since hydrocarbon is available in submicron micelles, it can be utilized by any cell, whether it is part of a floc or not. At high rates of agitation flocs are broken up as they become unstable, and the contribution of cells outside the flocs to hydrocarbon uptake could become significant, as flocculation is essentially o v e r c~m e .~.~,~ According to Blanch and Einsele4 attachment of cells to substrate droplets results in the formation of flocs. They reported that flocs were formed in the fermentor within 30 sec of the addition of hydrocarbon, and the flocs disappear upon complete consumption of the substrate. This suggests that the Mallee-Blanch model is for a system where floc formation is substrate induced. There have been reports5 of hydrocarbon-assimilating organisms that have the characteristics of clumping regardless of the substrate. Clumping or floc formation in fermentations involving these organisms would be characteristic of the organism, not the substrate. Also, there have been reports of organisms assimilating hydrocarbon without floc formation occumng. 6.10 In the case of Candida guilliermondii I F 0 0566, a hydrocarbonutilizing organism, no flocs were formed. Therefore flocs are not a necessary requirement for hydrocarbon uptake, a requirement of Mallee and Blanch's model. Furthermore, Mimura et aL6 reported an enhanced growth of C . gu...