Variations in total hardness, total alkalinity, concentration of Ca and Mg, color, dissolved organic matter, and the pH of the water and of the sub-aquatic soil have been recorded in 16 ponds throughout a period of 14 months. Most of the interrelationships between the chemical factors are related to the Ca content of the water. Seasonal fluctuations in Ca and total alkalinity occur in all the ponds studied. The degree of fluctuation is related to the concentration of dissolved organic matter in the water. The fluctuation is greatest in hard waters and certain small localities containing softer water. The color (measured as brownness) of the water is roughly proportional to the dissolved organic matter content, though a seasonal relationship between these 2 factors has been found only in 1 small, dystrophic pond. Erratic fluctuations occur most commonly in small bodies of water. The more important factors influencing the total numbers of mollusks, leeches, flatworms and Gammarus and Asellus present are Ca concentration, size of basin and concentration of dissolved organic matter. Few species are present in dystrophic localities. More species are found in oligo-trophic localities. The hard, eutrophic waters have a high content of dissolved organic matter and are rich in species and numbers of animals. It is suggested that one reason why so few animals occur in the soft-water ponds compared with larger bodies of water containing smaller amounts of Ca is that the concentration of humic acids tends to be much greater in the small basins.