The rotational spectrum of cyclopentadienylallylnickel, C 3 H 5 NiC 5 H 5 , has been studied using a pulsed molecular beam Fourier transform microwave spectrometer. Twelve a-type transitions were analyzed to obtain rotational and centrifugal distortion constants for the parent C 3 H 5 58 NiC 5 H 5 complex. The measured rotational constant A=3107.603(93)MHz is about 160.0MHz larger than the predicted DFT value, providing evidence for possible fluxional motion in the complex. The large distortion constants, on the order of 100kHz, provide further evidence for fluxional motion. The experimental constants B=1302.38(22) and C=1276.40(15)MHz are in good agreement with the DFT calculated values and confirm the η 3 -bonding of the allyl ligand to the Ni–C 5 H 5 moiety. DFT calculations provide a V 5 barrier for internal rotation about the Ni–C 5 H 5 axis of 53cm −1 , with the lowest energy conformation having the central allyl c-atom eclipsed with respect to two C 5 H 5 carbon atoms. Several additional rotational lines, possibly those of an exited torsional state, were observed but not assigned.