The Fermi surface has been compared for the three compounds LnB6 (Ln --- La, Ce and Pr) by measurement of the de Haas-van Alphen effect. The major part is observed to be similar in each compound, most likely consisting of X-centred ellipsoids with a total effective volume of about 1 electron per formula unit. Differences occur in the details of the shape of the FS, clearly seen for PrB6. Large effective masses, m*, are observed for CeB6 and PrB6, m* = 6 and about 1.8, respectively. These have to be compared with m* = 6.0 for LaB6.