One of the unique properties of biological PHA materials is their biodegradability in various environments. The rate of biodegradation of PHA materials depends on many factors, notably those related to the environment (temperature, moisture level, pH, and nutrient supply) and those related to the PHA materials themselves (composition, crystallinity, additives, and surface area). Electron microscopy has revealed that degradation occurs at the surface by enzymatic hydrolysis (surface erosion). The molecular weights of PHA samples remained almost unchanged during the course of biodegradation. A number of microorganisms such as bacteria and fungi in soil, sludge, and sea water excrete extracellular PHA-degrading enzymes to hydrolyze the solid PHA into water-soluble oligomers and monomer, and subsequently utilize the resulting products as nutrients within cells.