The round window niche entry (fossula fenestrae cochleae, fossula fenestrae rotundae) is one of the main structures in the labyrinthine wall of the tympanic cavity. Its variability is fairly significant and very important in otosurgery. In order to learn more about the morphology of the round window niche entry, a microanatomical study on 100 temporal bones obtained from cadavers aged 0-71 y. was performed. A standard set of otosurgical equipment was used. Five morphological forms of the round window niche entry were differentiated: triangular, semicircular, ovally horizontal, ovally vertical and round ones. Triangular forms were observed only in children of over 4 years of age and round forms almost only in young children, of age below this borderline (the beginning of pneumatization of the temporal bone). The differences between these two groups were statistically significant.