Benzenesulfonamide is a known inhibitor of carbonic anhydrase (CA). 8-[4-(Aminosulfonyl)phenyl]-BODIPY is designed as an inhibitor dye conjugate for turn-on fluorescence sensing of CA, and binds specifically to bovine carbonic anhydrase in water with K-a = 1.04 +/- 0.25 x 10(7) M-1 (K-d = 0.96 x 10(-7) M) with concomitant increase in the fluorescence intensity of the former by a factor of 2.5-2.8 because of the steric constraint imposed on the probe by the enzyme.