This study deals with the lenition of voiceless fricatives in two varieties of southern Italian. In the first part, we recall not only different ways in which the very notion of ‘lenition’ has been used in the traditional literature and in recent theoretical proposals, but we will also discuss the fortis vs. lenis distinction (as well as the related feature [±tense]), finally adhering to a rather narrow interpretation of the two concepts. A review of the available descriptions of southern varieties of Italian shows that an allophonic rule of lenition is widely reported as regards plosives, but only partially acknowledged for fricatives. The basic claim of our study is that this variable rule is best stated in terms of the feature [±tense] and thus applied to the class of obstruents as a whole, the acoustic correlates of this feature being mainly duration and eventually intensity. The empirical part of our study examines the occurrences of /s/ and /f/ produced by six male students (three from Naples and three from Palermo); the dialogues were recorded within the CLIPS project (Corpora e Lessici dell’Italiano Parlato e Scritto) according to the Map Task procedure. The results essentially confirm our hypothesis that lenition does indeed apply – even if to a variable degree – to fricatives as well; as to the phonetic implementation of the process, intensity seems not to be at stake at all, duration being its main acoustic correlate.