|Abstract:||In Czech, the postalveolar trill is realized phonetically as an obstruent but generally patterns as a sonorant with respect to voicing assimilation. This paper argues to treat such cases as instantiating phonological opacity, and describes the Czech case using the tools of Biaspectual Phonology (Bye 2005, Ms.). In BP, the phonological grammar interfaces with a phonetic interpretation (Phi) and a lexical recognition (Lambda) component, and different `aspects' of one and the same phonological representation may be visible to each system. Opacity becomes possible when what Phi sees and what Lambda sees do not completely match. The analysis is compared to other parallelist approaches to opacity and it is argued that the BP account is superior in two ways. First, the BP account is able to take full account of the richness of the base: the right output may be obtained regardless whether the postalveolar trill is sonorant or obstruent in the input. Second, the theory allows handling of rule sandwiching effects. In Czech, the rule sandwiching effect consists in the fact that the postalveolar trill behaves like a normal obstruent word-initially, but as a (crypto)sonorant elsewhere. To date, no parallelist alternative has been proposed that allows us to capture this kind of situation.