  Discontinuities in elastic constants are detected at the superconducting
transition of layered organic conductors \kappa-(BEDT-TTF)_{2}X by longitudinal
and transverse ultrasonic velocity measurements. Symmetry arguments show that
discontinuities in shear elastic constants can be explained in the orthorhombic
compound only if the superconducting order parameter has a mixed character that
can be of two types, either A_{1g}+B_{1g} or B_{2g}+B_{3g} in the
classification of irreducible representations of the orthorhombic point group
D_{2h}. Consistency with other measurements suggests that the A_{1g}+B_{1g}
(d_{xy}+d_{z(x+y)}) possibility is realized. Such clear symmetry-imposed
signatures of mixed order parameters have not been observed in other
superconducting compounds.
