Treatment of 6-chloro-1, 2, 3, 4-tetrahydrocarbazole (14) with the Vilsmeier-Haack reagent gave 6-chloro-2, 3, 4, 4a-tetrahydrocarbazole-4a, 9-dicarboxaldehyde (16) as a main product, along with 6-chloro-1, 2, 3, 4-tetrahydrocarbazole-9-carboxaldehyde (15) and 6-chloro-1, 2, 3, 4-tetrahydro-1-hydroxycarbazole-9-carboxaldehyde (17). The structures were established by spectral and chemical means. The mechanism of formation of 16 is discussed ; in connection with this, a similar reaction of 6-chloro-1, 2, 3, 4-tetrahydro-1, 1-dimethylcarbazole (37) was carried out and found to yield three products, 6-chloro-1, 2, 3, 4-tetrahydro-1, 1-dimethylcarbazole-9-(38), 5-(39), and 7-carboxaldehyde (40).