Baltic Sea algae were extd. with (i) aq. urea phosphate soln. or (ii) H2O under ultrasound irradn. The exts. were tested for antibacterial activity against both Gram positive (Staphylococcus aureus) and Gram negative (Escherichia coli) bacteria. The exts. obtained under (i) showed bacteriostatic activity but that under (ii) accelerated the bacterial growth.