Paracoccus denitrificans accumulated a polyester in its cells during growth on n-pentanol. The composition of the polyester varied during the cultivation: the level of the 3-hydroxyvalerate unit in the polyester increased, and eventually a homopolymeric poly(3-hydroxyvalerate) [P(3HV)] accumulated to an amount 22 to 24% of the cell dry weight. Growth-associated polyester synthesis was considerably affected by n-pentanol when its concentration was controlled at several levels. Maximum accumulation of the polyester was obtained at 0.02% (vol/vol). Physical and mechanical characteristics of the P(3HV) were determined and compared with those of other homo- and copolyesters. The P(3HV) was dextrorotatory and had number-averaged and weight-averaged molecular masses of 128,000 and 888,000 Da, respectively, with a rate of polydispersity of 6.93. The level of tensile strength of the P(3HV) was lower, and its extension to break was higher than that of the poly(3-hydroxybutyrate) homopolyester.