The aim of this study is presentation of chosen chemical proprieties of mud soils and comparison of them with chemical proprieties of peat soils formed from different kinds of peats. The researches were led in the Narew and the Biebrza flood valleys created in the Lower Biebrza Basin. The results of the researches show the statistically significant differences in chemical composition between mud and peat soils. Muds contain less total nitrogen (average 1.633% d.m.) and calcium in ash (average 2.297% d.m.), but they are richer into potassium (average 0.198% d.m.) and sodium (average 0.030% d.m.). However, they do not differ from peats as regards of content of magnesium (average 0.316% d.m.), iron (average 2.923% d.m.), aluminium (average 2.772% d.m.) and phosphorus in ash (average 0.134% d.m.). Content of total nitrogen and phosphorus in mud soils get smaller far into soil profile. Limnethic muds are more fertile than telmathic muds, because they have more total nitrogen, calcium, iron and phosphorus in ash. Proper muds have larger content of total nitrogen, potassium, sodium, calcium, magnesium, iron and phosphorus in ash in comparison with silted muds. Fertility of mud soils is caused first of all by mud forming process and in smaller degree by silty process.