Among 916 Pseudomonas aeruginosa strains isolated from children in a Warsaw hospital during the period 1985-1988 the most frequently encountered serotypes were O6, O11, O12, and O16. The majority of isolates resistant to aminoglycosides were characterized by simultaneous resistance to gentamicin and tobramycin and belonged to serotype O11. Among isolates resistant to beta-lactam antibiotics the most frequently encountered serotype was O6, followed by serotype O3 and O11. Most strains of serotype O12 were resistant to aminoglycosides and beta-lactam antibiotics as has been reported from other countries.