Summary. 21-cm observations of 11 dark clouds were made using the Jodrell Bank Mk1A telescope. Self-absorption was detected definitely in six. Lower limits to the 21-cm column densities are derived and the average value is found to be a factor of 25 lower than the average value of Knapp. The 21-cm column densities are compared with the column densities of molecular hydrogen estimated from CO observations, and lower limits for the ratio N(H i)!N(H2) range from 0.8 to 3.3 x l0- - Estimates of the cosmic-ray ionization rate deduced from these observations suggest that destruction of H2 by low-energy cosmic rays can adequately account for the presence of atomic hydrogen in dark clouds.