The marine pseudomonad UP-2 grew, with a doubling time of 1.5 h at 30°C, in media supplemented with n-tetracosane as the sole organic carbon and energy source. Over 99% of the solid hydrocarbon was degraded. In order to overcome serious problems of sampling and measuring growth in the biphase system of hydrocarbon-water, a technique was developed combining the use of separate 1-ml eultures for each sample and32P labeling as an assay for growth. Using this technique, a growth durve of UP-2 on n-tetracosane was obtained. UP-2 pregrown on sodium acetate can utilize even-chain, liquid, or solid n-alkanes with no observable lag; in contrast, when UP-2 was pregrown on n-tetracosane and transferred to sodium acetate medium, a lag of 1 h was observed.