SummaryBetween February 1983 and December 1987, 262 invasive isolates ofHaemophilus influenzaewere sent to the national reference laboratory in Belgium. Type b strains were more commonly isolated from children (95 %) than from adults (55%) (p<0.0001). The prevalence of (lactamase production significantly increased from 18.5 % in 1983 to 35 % for the period 1986–1987 (p<0.05) and was somewhat higher among type b strains (29.5 %) than among other isolates (22.5 %). A strong correlation was found between resistance to ampicillin, chloramphenicol and tetracycline. Eleven B-lactamasc producing strains (17%) were resistant to chloramphenicol (p<0.001); all 13-lactamase negative strains were susceptible to chloramphenicol. Ampicillin resistant strains lacking 6-lactamase activity were not found. Monitoring the antimicrobial susceptibility ofH. influenzaeis an important component of a surveillance system forH. influenzaeinfections.