To compare, in vitro, the effect of placing opaque (OPS) and clear fluorescing (CFS) pit and fissure sealants (PFS) on the detection of occlusal caries (OCD).Occlusal surfaces of 67 extracted molars were examined under standardised conditions by 6 final year undergraduate dental students, using visual, bitewing radiography, transillumination (FOTI), laser fluorescence (LF) and tactile methods of caries detection. The teeth were then assigned randomly to two groups for PFS placement: OPS and CFS; then the OCD methods were repeated. Caries presence/absence was determined histologically on serial sections examined under stereo-microscopy (10x).Before PFS placement the sensitivity and specificity for the OCD methods were: visual: 68%, 71%; radiographic: 15%, 95%; FOTI: 36%, 93%; LF: 49%, 83% and tactile: 39%, 67%, respectively. After placement of OPS, the sensitivity of LF (20%) and visual (13%) methods decreased and specificity increased (93%, 98% respectively). Placement of CFS resulted in minor changes in sensitivity and specificity. Correlation (Spearman's Rho coefficients) between OCD methods and histological intra-dentinal caries for pre- PFS, OPS, and CFS were: visual: 0.38, 0.34, 0.33; FOTI: 0.42, 0.35, 0.43; and LF: 0.41, 0.30, and 0.45 respectively.The sensitivity of all OCD methods was low, as well as their correlation to the histological gold standard. Placing OPS further decreased the sensitivity of LF and visual methods, whereas placing CFS had little effect on all OCD methods. It is recommended that tactile detection of occlusal caries should be discontinued, and the probe used only to clean the pits and fissures gently for more accurate visual detection, or prior to pit and fissure sealant placement. Further research into the development of an affordable, robust, accurate and easy to use method for OCD is required.