Coagulation and gravitational settling are two of the most important physical processes affecting the fate of particulate matter in natural aquatic environments. Mathematical models of these two processes typically assume that particles coalesce into spheres upon sticking. However, natural aggregates are more often fractal in character, and this aspect of aggregate structure can dramatically affect both the coagulation and settling process. In this paper, we investigate the steady-state coagulation and settling of fractal aggregates in water. A family of solutions is found for a specific form of the coagulation kernel, and a more general ''scaling theory'' is presented for any coagulation kernel that is a homogeneous function of its arguments. From this scaling theory, simple power-law expressions are derived for the depth (z) evolution of the total aggregate concentration (N t 8 ~z - 7 ), the particle volume fraction (φ~z - ε ), and the average aggregate volume (ν~z γ - ε ). The exponents γ and ε in these expressions can be calculated for any particular system from the fractal dimension (D) of aggregates undergoing coagulation and the homogeneity constant (λ) of the coagulation mechanism. These results are used to analyze the particle volume distributions published previously for Lake Zurich, Switzerland [U. Weilenmann, C.R. O'Melia and W. Stumm, Linmol. Oceanogr., 34 (1989) 1]. We find that the total aggregate concentration data can be fit to a power-law expression, and that the resulting exponent is consistent with a degree of homogeneity for coagulation in the lake between diffusion-limited (λ=0) and reaction-limited (λ=1) aggregation. The volume fraction data also can be fit to a power-law expression, but the resulting homogeneity constant is much larger (λ>1.4), perhaps reflecting the removal of large aggregates from the water column by filter feeding organisms.