Whole-word frequency effects are shown to exist in what appears to be a completely regular system, the spelling of prefix-final /z/ in Russian. Russian prefixes that underlyingly end in /z/ ( roz -, bez -, iz -) end in [s] on the surface when followed by a voiceless consonant. According to the rules of Russian orthography, the surface form, rather than the underlying form, must be reflected in the spelling. However, spelling errors reflecting the underlying form often occur, especially for the prefix bez -. The present paper reports that the error rate, either in natural typing on the web or in a classroom dictation task, for a given word is negatively correlated with the frequency of the word, suggesting that Russian writers rely, to a significant extent, on memory of complete orthographic forms as opposed to the orthographic rule. The frequency effect holds even within the set of regular inflectional variants of a single lexeme, with more frequent wordforms showing lower error rates. The evidence demonstrates a high degree of reliance on whole-form lexical retrieval even in what appears to be a regular system that is explicitly taught to the writers throughout their schooling in a morphologically rich language and thus provides support for the use of lexical retrieval even when it is not necessary (Baayen et al., Dutch inflection: The rules that prove the exception, Kluwer, 2002, Butterworth, Lexical representation, Academic Press, 1983, Bybee, Morphology: A study of the relation between meaning and form, John Benjamins, 1985, The phonology of the lexicon: Evidence from lexical diffusion, CSLI, 2000, vs. DiSciullo and Williams, On the definition of word, MIT Press, 1987, Pinker, Science 253: 530–535, 1991). However, reliance on retrieval is argued to be especially strong when there is a relatively long period of temporary uncertainty regarding which rule is applicable during processing (see also Albright, Lexical and morphological conditioning of paradigm gaps, Equinox, 2009, Barca et al., Reading and Writing 20: 495–509, 2007, Burani et al., Psychonomic Bulletin and Review 13: 346–352, 2006). The importance of temporary uncertainty and resulting rule competition suggests that the regular/irregular distinction needs to be reconsidered as even fully “regular” systems may feature rule competition due to temporary uncertainties about rule applicability. Reliance on retrieval may go largely undetected in Russian during schooling because teaching and test materials focus on the spelling of frequent words, which can be either produced by rule or retrieved as wholes. The largely complementary methodological challenges in studying lexical frequency effects in corpus and experimental data are discussed.