The position of carious lesions in fissures of 50 maxillary premolars was studied in serial ground sections in polarized light using air and various aqueous media for imbibition. The particular section in which the greatest pore volume was demonstrated, was selected for morphometric measurements of the fissure, i.e. depth, width, and angle. The localization of the carious lesion was registered in relation to four areas around the fissures, i.e. above the entrance, the top and bottom part of the wall, and below the base of the fissure. Most frequently the carious lesion was localized in the lower part of fissures (61%). Multiple foci both in the upper and lower part were found in 36% of the fissures. Only 13% had carious lesions positioned above the entrance. No relationship was found between fissure morphology and site of the carious lesions. However, this study has confirmed that fissured areas are highly susceptible to caries in premolars.