A description is given of a program of comparative thermal conductivity tests made in seventeen different laboratories on corkboard at mean temperatures from 20 F to 130 F with 20 different guarded hot plates. The objective of the program was to determine the reproducibility of results obtained in tests made in accordance with the requirements of ASTM Method of Test C 177-45. Examples of results showing good and poor agreement are presented and discussed. Histograms are given showing the frequency distribution of the average differences between the results obtained with each apparatus and the corresponding results obtained with a reference apparatus.