The fluorenylidene-substituted phosphaalkene Mes*P=Fl* (13) and arsaalkene Mes*As=Fl* (14) (Mes* = 2,4,6-t-Bu3C6H2, Fl* = 2,7-di-tert-butylfluorenylidene) have been prepared by treatment of Fl*(H)Li with Mes*ECl2 (E = P, As) followed by dehydrochlorination with DABCO (1,4-diazabicyclo[2.2.2]octane). In the synthesis of 13 it was possible to isolate the intermediate, Mes*(Cl)P-(H)Fl* (11). Attempts to prepare the stibaalkene, Mes*Sb=Fl* (15), resulted in an unstable product. Compounds 11, 13, and 14 were characterized by X-ray crystallography: 11, triclinic, , a = 11.548(2) Angstrom, b = 12.189(3) Angstrom, c = 14.223(3) Angstrom, alpha = 114.13(1)degrees, beta = 92.79(1)degrees, gamma = 107.14(1)degrees, V = 1712.8(2) Angstrom(3), Z = 2, wR2 = 0.1344, R1 = 0.0560; 13, monoclinic, P2(1)/n, a = 24.597(7) Angstrom, b = 11.185(2) Angstrom, c = 26.914(7) Angstrom, beta = 112.16(2)degrees, V = 6857(3) Angstrom(3), Z = 8, wR2 = 0.1477, Fl = 0.0794; 14 . Et2O, monoclinic, P2(1)t/n, a = 15.577(4) Angstrom, b = 15.274(4) Angstrom, c = 17.255(5) Angstrom, beta = 108.17(2)degrees, V = 3901(2) Angstrom(3), Z = 4, wR2 = 0.1275, R1 = 0.0578. The P=C (1.686(5) Angstrom) and As=C (1.807(3) Angstrom) bond distances in 13 and 14 are consistent with double-bonding descriptions; moreover, there is a zero angle of twist between the E-C(Mes*) moieties and the plane of the Fl* ligand in both molecules.