The work presented in this paper was initiated by recent efforts to use circumferential creeping waves for ultrasonic nondestructive testing of inaccessible airframe structures for fatigue cracking. One particular such application of great interest in the aerospace industry is the inspection of weep holes drilled through vertical stiffeners in wet-wing structures (used as fuel tanks) of both military and civilian aircraft in order to permit remnant fuel to be evenly distributed during flight. Unfortunately, they can become the sites from which fatigue cracks often originate. A new split-aperture technique was recently suggested for the inspection of these holes [1]. The outstanding sensitivity of the split-aperture circumferential creeping wave technique in dry weep holes was well demonstrated on both simulated and real fatigue cracks. However, the presence of fuel or even its residues in the weep hole significantly affects the behavior of the circumferential creeping wave. It causes the creeping wave echo to split into a rather weak Rayleigh-type surface wave signal and a much stronger but somewhat slower halo signal [2]. In order to adapt the circumferential creeping wave inspection technique to the case of fluid-filled cavities it is important to understand the adverse effects of fluid loading on the Rayleigh- type circumferential surface wave, which is the only mode sensitive to such cracks.