We have examined heteroflocculation in binary colloidal suspensions by Monte Carlo simulations based on a diffusion-limited-cluster-aggregation (DLCA) model modified with finite attraction energies, The simulations were performed in two dimensions, Under heteroflocculation conditions, i.e., attraction between unlike particles and repulsion between like particles, cluster size undergoes a maximum as the concentration of the second species of particles is increased, similar to the experimental results in both binary suspensions and suspensions with adsorbing polymers. The initial increase in cluster size at low concentrations of the second species of particles is due to the mutual attraction between unlike particles, The decrease in cluster size at higher concentrations of the sea,nd species of particles is due to the repulsion between the second species of particles, The distinction between heteroflocculation and particulate depletion flocculation is discussed.