In contemporary Finnish literature informal and colloquial varieties of language are widely used for stylistic purposes. This study investigates the use of colloquialisms in translated Finnish fiction as compared to non-translated texts of the respective genres. Due to the corpus-based approach on a fairly large material, the primary emphasis is on quantitative aspects, but some attention is paid to qualitative differences as well. The study is descriptive in nature and is restricted to lexical manifestations of colloquialisms, i.e., non-standard spelling and vocabulary. Based on the presumption that translations are conservative in comparison to original texts, it is hypothesised that both the extent and the frequency of colloquialisms are greater in original fiction than in translated fiction. Colloquialisms are examined using fourteen groups of words established with the help of corpus-based key word lists.