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There is ample observational evidence that cluster galaxies are different from those in the field. Interaction with the hot intracluster medium affects the morphology of the galaxies, their gaseous content and possibly their star-formation activity. Tidal encounters between galaxies also play an important role.The atomic component has been investigated in detail for several clusters, among them our neighbour Virgo. A large fraction of the bright Virgo spirals have about one order of magnitude less HI than isolated counterparts of same optical size and type (Giovanelli & Haynes : 1983, AJ 88, 881). What about the molecular phase of the interstellar medium in cluster galaxies ? Molecular clouds are the raw material for star formation. CO surveys of Virgo bright galaxies (Stark et al. : 1986, ApJ 310, 660 ; Kenney & Young : 1989, ApJ 344, 171) do not show any evidence of H2 deficiency : the mechanisms tearing out the low-density atomic gas leave intact the denser molecular clouds. A similar result has recently been found for the much denser and more distant Coma cluster (Casoli et al. : 1991, A&A 249, 359). To date, CO observations are available for those two rich clusters only.With the Swedish-ESO 15 m telescope, we have observed in the 12 CO (1-0) transition the 23 brightest spirals and lenticulars of the Fornax cluster. Fornax is the nearest southern cluster of galaxies, at the same distance as Virgo (D = 17 Mpc adopted, consistent with H 0 = 75 km s -1 Mpc" 1 ). Fornax contains 7 times less galaxies than Virgo, but its central regions are 2.5 times denser. The spatial resolution of the CO observations is 44", corresponding to a linear distance of 3.6 kpc at 17 Mpc. Every galaxy with an optical diametre larger than 3' was mapped at half beamwidth along the major axis, even if no CO was detected towards the centre. Since every extended galaxy of the sample happens to have a high inclination, we expect not to miss any CO emission. With the Nan9ay radiotelescope, we also measured the HI emission of 11 galaxies of our sample. The resolution of the HI observations is 4' (E-W) by 21' (N-S).Only 9 galaxies out of 23 were detected in CO, despite long integration times (typically 80 minutes) and the low noise level (we could detect H2 masses as low as 5 10 7 M 0 ). The average CO emission of Fornax galaxies is 8 times lower than in a template sample based on the FCRAO extragalactic survey. Figure 1 displays the M(H 2 )/D 2 ratio for the Fornax (triangles) and template (circles) galaxies versus their morphological type. 174https://ntrs.nasa.gov/search.jsp?R=19930017602 2018-05-10T16:25:54+00:00Z