The effect of internal waveson linear vortices in a homogeneous bounda
ry layer is examined by taking a simple model with a horizontal array
of line vortices or vortex cells lying in a layer bounded above by a r
igid plane and below by a density interface on which interfacial waves
are free to propagate. The interfacial waves stretch, compress, and d
isplace the vortices, so changing their vorticity, orientation, and se
paration by amounts that are estimated. As a consequence, the instabil
ity of an array oi vortices of alternating signs is enhanced in region
s that depend on the local phase of the interfacial waves. The vortice
s force secondary disturbances on the wave-perturbed density interface
. For parameter values typical of the ocean, the potential energy asso
ciated with these disturbances may be comparable with the kinetic ener
gy in the vortices. The energy required to drive the vortices is there
fore greater than that in the absence oi internal waves, and this may
affect the growth and development of the vortices. The presence of a d
ensity interface at the loot of the mixed layer however, increases the
primary rate of growth of Langmuir circulation in comparison with tha
t found when the lower boundary is rigid. The subsequent instability i
s also enhanced. In consequence Langmuir cells in mixed layers overlyi
ng stratified water are expected to grow more rapidly and to be more u
nstable than those developing in a homogeneous layer of the same depth
overlying a rigid bottom. The effect of codirectional shear and Stoke
s drift included in the Craik-Leibovich equations is to reduce the pha
se speed of internal waves that propagate normal to the mean flow.