The reaction of N-phenylbenzimidoyl isoselenocyanates 8 with primary and secondary amines in acetone at room temperature, followed by treatment with a base, led to 6H-[5,1,3]benzoselenadiazocine derivatives of type 10 (Scheme 3). An analogous cyclization was observed when 8a and 8b were reacted with the Na salt of diethyl malonate in EtOH at room temperature, which yielded the eight-membered selenaheterocycles 11 (Scheme 5). The molecular structures of some of the products, as well as that of a sulfur analogue, have been established by X-ray crystallography (Figs. 1 - 4). The isoselenocyanates 8 have been prepared from N-(2-methylphenyl)benz-amides 5 in a three-step procedure via the corresponding imidoyl chlorides 6, side-chain chlorination to give 7, and treatment with KSeCN (Scheme 2).