The representation of affricates remains an unsettled issue in phonology. The main question is whether affricates should be treated as simple or complex segments. In the former view, affricates are stops specified for an additional feature, such as [strident]; in the latter, affricates are complex segments specified for [+continuant] and [–continuant] in an unordered fashion. Using the framework of Optimality Theory, this paper investigates similarity avoidance effects in Basque under the two competing theories of representation. The posited OCP effects involve the avoidance of [strident] and [–continuant] clusters. Importantly, the complex segment approach requires the use of two additional constraints militating against [+continuant] and [+obstruent] clusters. The analysis successfully accounts for the imperfective formation data as well as for other processes in Basque, such as simplification of stop clusters, stop+fricative coalescence and lenition of voiced obstruents. Additionally, the article discusses asymmetries in consonant mutations connected with the OCP effects. In particular, the preference of surface fricative+stop clusters over stop+fricative clusters is attributed to a high-ranked markedness constraint, NOAFFRICATECONTRAST. An important conclusion of the analysis is that the complex segment approach creates unnecessary complications, as the parallel framework allows to maintain satisfactory generalizations within the simple segment approach.