Alginate beads containing a denitrifying isolate (Pseudomonas sp.) and starch were pretreated by freeze-drying before incubation under denitrifying conditions. The physical and denitrifying properties of this immobilization complex were compared to those of untreated 'wet' alginate beads. Freeze-dried alginate beads revealed physical properties similar to those of porous, spongelike matrices. As compared to conventionally prepared alginate beads, physical damage, due to gas accumulation under denitrifying conditions, was considerably reduced in freeze-dried beads. Stress-strain tests were performed, and the latter beads were found to be considerably stronger than the conventionally prepared ones. The stronger, freeze-dried beads sustained denitrifying activity ever a prolonged period as compared to the regular beads.