Carbohydrate-binding modules (CBMs) are often part of the complex hydrolytic extracellular enzymes from bacteria and may modulate their catalytic activity. The thermostable catalytic domain of laminarinase Lam16A from Thermotoga neapolitana (glycosyl hydrolase family 16) is flanked by two CBMs, 148 and 161 aa long. They share a sequence identity of 30%, are homologous to family CBM4 and are thus called CBM4-1 and CBM4-2 respectively. Recombinant Lam16A proteins deleted for one or both binding modules and the isolated module CBM4-1 were characterized. Proteins containing the N-terminal module CBM4-1 bound to the soluble polysaccharides laminarin (1,3-beta-glucan) and barley 1,3/1,4-beta-glucan, and proteins containing the C-terminal module CBM4-2 bound additionally to curdlan (1,3-beta-glucan) and pustulan (1,6-beta-glucan), and to insoluble yeast cell wall beta-glucan. The activity of the catalytic domain on soluble 1,3-beta-glucans was stimulated by the presence of CBM4-1, whereas the presence of CBM4-2 enhanced the Lam16A activity towards gelatinized and insoluble or mixed-linkage 1,3-beta-glucan. Thermostability of the catalytic domain was not affected by the truncations. Members of family CBM4 can be divided into four subfamilies, members of which show different polysaccharide-binding specificities corresponding to the catalytic specificities of the associated hydrolytic domains.