The extremely sensitive deposits of post-glacial marine clay in the St. Lawrence lowlands have presented major difficulties in obtaining adequate undisturbed samples for a reliable determination of shear strength. To overcome this problem, a simple field vane apparatus was devised which was readily adaptable to existing drilling equipment. This paper describes the apparatus and outlines the method of testing that has been used in the field. Shear strength values obtained are compared with those obtained from tube samples from several test holes of Leda clay. The influence of such factors as the area ratio of the vane and rod friction are discussed along with its use as a measure of sensitivity. It is concluded that the field vane device offers a more reliable criterion for shear strength of the clay than determinations made from tube samples. Advantages and restrictions on its use are also considered.