The rotational spectra of four isotopologues of the 1:1 complex between formic acid and pyridine show that the two units are linked together through a "classical" (OH center dot center dot center dot N) and a weak (CH center dot center dot center dot O) hydrogen bond. The molecular system appears quite rigid and no effects of the internal motions have been observed in the spectrum. The dissociation energy obtained from the centrifugal distortion by applying an approximate model, 39.8 kJ/mol, is quite similar to the ab initio value, 41.7 kJ/mol. Its relatively high value suggests a small charge transfer to take place.