Thermally synthesized poly(aspartate) (tPAA) shows potential for use in a wide variety of products and applications as a biodegradable replacement for non-biodegradable polycarboxylates, such as poly(acrylate). The tPAA molecule has unnatural structures, and the relationship between its biodegradability and structures has been investigated. Two tPAA-degrading bacteria, Sphingomonas sp. KT-1 and Pedobacter sp. KP-2, were isolated from river water; from them, two PAA-hydrolyzing enzymes, PAA hydrolases-1 and -2, were purified and biologically and genetically characterized. Interestingly, not only are PAA hydrolases-1 from those two strains novel in terms of structural genes and substrate specificities (they specifically cleave the amide bond between β-aspartate units in tPAA), they also probably play a central role in tPAA biodegradation by both strains. In green polymer chemistry, one active area of research is the use of purified enzymes for the enzyme-catalyzed synthesis of polypeptides by taking advantage of their substrate specificities. Recently, β-peptides have attracted academic and industrial interest as functional materials as they possess both functions of α-peptides and excellent metabolic stability. As one of the attractive applications of PAA hydrolases, we report here the enzyme-catalyzed synthesis of poly(α-ethyl β-aspartate), which is composed of only β-linkages and belongs to β-peptides, using the unique substrate specificity of the enzyme from Pedobacter sp. KP-2.