Kinetic fractal aggregation in a particle bath where a fraction f of the sites are initially occupied is studied with d=2 computer simulations. Independent particles diffusing to a fixed cluster produce an aggregate with fractal dimension D≅ 1.7 up to a correlation length ξ(f). At larger lengths D→2. ξ(f) → ∞ as f → 0. When the particles remain fixed but the cluster undergoes a rigid random walk D appears constant at larger scales but varies with f. D → 1.95 at large f and D → 1.7 as f → 0. In both cases, the aggregate size N(t) grows with time tγ(f) . Aggregation on a surface by independently diffusing particles produces shapes reminiscent of electrochemical dendritic growth. The dependence of growth rate and geometry is studied as a function of particle concentration and sticking probability. © 1984 Plenum Publishing Corporation.