A theoretical study of fully developed heat transfer for in-line slug flow through unbaffled equilateral triangular bundles is reported. Results are given for the pitch: diameter range 1.05 to 2.00. Two sets of thermal boundary conditions have been considered: (a) uniform wall heat flux in all directions and (b) uniform wall heat flux in the axial direction and uniform wall temperature in the circumferential direction. For the first set, results on the circumferential variation of the wall temperature are given; and for the second, those on the circumferential variation of the wall heat flux are given. For both sets, average Nusselt numbers and circumferential variations of the local heat-transfer coefficients are also given. In all cases, the results are presented in the form of convenient dimensionless groups, and it is shown that they apply to, or can be used for, the estimation of the same dependent variables for turbulent flow of liquid metals through rod bundles. It has also been shown that for the P/D ratios and Peclet numbers normally employed in liquid-metal-cooled reactor cores, the ratio of the maximum temperature variation around a rod to the average wall-to-bulk temperature drop, in the case of uniform wall heat flux in all directions, is not greatly different for both slug and turbulent flows.