The findings of a theoretical study of heat transfer for laminar, in-line flow through unbaffled rod bundles are reported. The results of a numerical solution are given for equilateral triangular bundles, for P/D ratios ranging from 1.001 to 2.00, for fully developed temperature profiles, and for the thermal boundary conditions of uniform wall heat flux in all directions. They are given in terms of rod-average heat transfer coefficients and circumferential variations of the wall temperature. The rod-average heat transfer coefficient goes through a rather sharp maximum as the P/D ratio is varied, the maximum occurring at P/D = 1.20. The circumferential variation of the wall temperature, large at small P/D ratios, decreases as P/D is increased, until at P/D > ∼ 1.50 it is negligible. Results calculated for the thermal boundary conditions of uniform wall heat flux in the axial direction and uniform wall temperature in the circumferential direction agreed excellently with previous results, attesting to the accuracy of the present calculational method.