Oryginality and objective – Research on the pricing of stocks listed on developed markets shows inexplicable deviation from the pricing that could be observed with CAPM validity. A similar anomaly is found on the Polish market. Reasons for inconsistent pricing with CAPM are unknown, and they are the main objective of this research.

Method – The study is conducted using stocks listed on the Warsaw Stock Exchange in 1995–2012. Quintile stock portfolios are formed on the basis of strategies widely used by investors. The study is carried out in several modes. In the subsequent modes penny stocks with the market values below 0.5, 1.5, 5.0 and 15.0 PLN are eliminated.

Results – It is conjectured that both penny stocks and improper procedures for the test portfolios forming contribute to inconsistent stock pricing in light of the CAPM. The studies show that results are in line with the extended conjectures. Also, study results indicate that speculative stocks are mostly penny stocks, however, it is not possible to explicitly state that penny stock are speculative.