  We report measurements of the de Haas-van Alphen (DHVA) effect and AC
susceptibility in the layered organic superconductor
$\kappa$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$. The amplitude of the DHVA oscillations
is attenuated over a wide field range above the irreversibility line, B_irr(B),
below which a rigid flux lattice is formed. Thus the DHVA effect provides a
unique probe of the superconducting fluctuations at high fields in this
material. We compare our measurements with other determinations of the
superconducting phase diagram of $\kappa$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$.
