This chapter determines the biodegradability of various poly (beta-hydroxyalkanoate)'s (PHA) and the mechanism of their biodegradation. These experiments have employed the turbidometric assay and will be broadened to include other techniques such as determination of particle size changes, molecular weight changes, and changes in crystallinity. A variety of PHA's depending on the species of bacterium and the compounds supplied as carbon sources for growth. There are a number of potential applications for polymer materials, both in medicine and for industrial applications. Biodegradability of PHA's will be tested using the purified, extracellular depolymerases of P. lemoignei. The three carbon backbone was found to be required for appreciable enzymatic activity, and other biodegradable polyesters with a different number of carbons in the repeating unit were not hydrolyzed by this enzyme. One of the important aspects of the microbiology rule involves stereochemistry. The initial rate of degradation is linear over the first 9 to 15 minutes then slows with time.