A new expression system was developed for Bacillus subtilis. This system uses a shuttle vector (B. subtilis - Eschericia coli) carrying a phosphate starvation-inducible promoter (pst) and on a fed-batch cultivation strategy. The pst-promoter proved to be very strong and retain its tight regulation also when present on a multi-copy plasmid. The expression system developed showed promising results when applied to the production of recombinant Bacillus phytase - phytase activity at the end of cultivation reached 28.7 U ml(-1).