According to the texton theory (Julesz, 1984a, 1984b), preattentive perception is mediated by the detection of density differences of features known as textons. The present study examined this assumption in the case of textons defined by ends of lines (terminators). This was done using a reaction time task in which subjects searched for a target of high terminator density embedded in arrays of lower terminator density. Results indicated that if density differences are utilized in preattentive target detection, then preattentive perception occurs only for particular highly practiced subjects, and then only for large density differences. When terminators are preattentively detected, observers appear to more typically detect only their presence, without computing density differences per se. Implications for other classes of textons were also considered.