Heterogeneity of beta-lactamase production by 17 clinical and nine environmental isolates of Stenotrophomonas maltophilia was investigated using MICs of six different beta-lactam antibiotics, isoelectric focusing (IEF) and pulsed field gel electrophoresis. There was no clear correlation between the results of IEF, genotype and MIC determination. Environmental isolates were more susceptible than clinical isolates; eight clinical and none of the environmental isolates expressed high-level resistance to meropenem. Only two isolates expressed high-level resistance to ceftazidime. These results indicate that further studies are required to elucidate the extent of genetic heterogeneity within the L1 and L2 beta-lactamase genes.