Polyribonucleotide phosphorilase from the psychrophilic Antarctic eubacterium Pseudoalteromonas haloplanktis (PhPNPase) has been purified. This enzyme catalyzes both the RNA polymerisation and degradation reaction, showing the highest activity at temperatures below 40 degrees C. PhPNPase is quite sensitive to heat treatment and it is endowed with remarkable halotolerance.