In a recent paper, we described the development of a method for calculating exact collision probabilities between different regions (namely, fuel kernels, graphite matrix, moderator, and coolant) of a lattice cell of a high temperature reactor (HTR) of the pebble bed variety. The method was shown to adequately represent the double heterogeneity in such reactors. In the present paper, we use some of the results obtained in that paper to construct a fast Monte Carlo algorithm for treatment of HTRs. This paper discusses the theoretical basis of the Monte Carlo algorithm, its implementation for the case of a lattice cell with the energy variable treated using a multigroup library, and results obtained. The method can be easily extended to full-core calculations using point cross-section data.