The present paper deals with the calculation of the thermal flux in the moderator of a supercell containing several fuel and control elements. It is assumed that the one group diffusion equation with a constant source holds in the moderator. It is further assumed that the absorption of thermal neutrons in fuel rods (or control rods) may be described as due to a cylindrically symmetric line sink. With these assumptions the diffusion equation is solved and a general expression for the thermal flux in the moderator is obtained. This expression is then used to calculate the thermal utilization in the supercell, and the ratios of absorption rates in the different elements of the lattice. General expression for the flux ratios and thermal utilization are obtained. By way of illustration, solutions of two typical cases of complex lattices are given