Studies were undertaken on the relationship between egg size and duration of embryogenesis in four freshwater fish species (Salmo trutta L, Esox lucius L., Gasterosteus aculeatus L., Leucaspius delineatus Heck.). Analyses were carried out in a comparative sense (different taxa) as well as with respect to eggs of the same species, or even laid by the same female. In addition to understandable differences of egg size between particular species, considerable differences were observed as regards egg size in fish of the same species, and even originating from the same female. In extreme cases the differences in egg size of the same female reached up to 11 percentage points (linear size) and over 40 (volume)! A strict relationship was found between duration of embryonic development and egg size: the smaller the egg, the shorter its embryogenesis. Biological meaning of this phenomenon has been discussed.