Poly(ester-ether-ester) triblock copolymers composed of poly(ethylene glycol) (PEG) and lactones, D,L-lactide or epsilon-caprolactone, were crosslinked with beta-chitin to prepare semi-interpenetrating polymer network (IPN) hydrogels by the u.v. irradiation method. Triblock copolymers were synthesized by bulk polymerization using low toxic stannous octoate as catalyst, or without catalyst. Photocrosslinked hydrogels exhibited an equilibrium water content in the range of 60-77%. From differential scanning calorimetry (d.s.c.) analysis, all the hydrogels revealed a drastic decrease in crystallinity after photocrosslinking. In the swollen state, tensile strengths of the semi-IPN hydrogels ranked above 1 MPa. In addition, in spite of their relatively high mechanical strength, elongation at break of swollen hydrogels ranged between 30 and 70%.