SCIENTIFIC ABSTRACT: This proposal is a part of a coherent program to study the interstellar medium in selected normal (non-interacting, non-starburst, non-AGN) galaxies in the near- to far-IR in coordination with the PHOT and SOT consortia. The present part of the program consists in imaging a complete sample of spiral, irregular and blue compact galaxies in the Virgo cluster in two filters: LW 2 (5.0-8.0 micrometers) and LW 3 (12-18 micrometers). The emission in LW 2 will be dominated by IR emission bands and that in LW 3 by hot dust. The nature and physics of both components is not well understood and the IRAS 12 micrometer filter did not allow a clear separation between them. However IRAS and other observations have shown that the intensity of both are affected by the UV radiation field and by metallicity, and also that the IR band carriers (PAHs or related large molecules) are easily frozen on bigger grains or liberated from them. Together with the other coordinated ISO observations and complementary gound-based observations they will yield a more complete picture of the physics of dust and possibly a full understanding of the thermal balance of dust and gas in galaxies. As some properties of galaxies e.g. their HI content depend on their environment the proposed observations of a complete sample of galaxies in the center and in the outskirts of the Virgo cluster will allow mapping and statistical studies of the global properties of galaxies (both spiral and irregular) in the mid-IR as well as a study of the effect of the environment. These galaxies will also serve as templates for ISO studies of clusters at large distances (see the corresponding ISO core-programme proposals). All the selected Virgo galaxies will also be observed by PHOT (PHOT guaranteed time) and the brighter half by LWS in the 158 micrometer line of ionized carbon (SOT guaranteed time). OBSERVATION SUMMARY: Full imaging of the target galaxies roughly up to the Holmberg radius will be done with 6" pixels. Some galaxies are considerably more extended than the field of ISOCAM. With filter LW 2 we will use microscanning in order to have a good flat field thus a high, uniform sensitivity (about 1 microjansky per arc second rms) in spite of the fairly high zodiacal background. With LW 3 we will use position-switching yielding a sensitivity of about 3 microjansky per arc second, consistent with the 3-5 times larger flux expected in this filter. However in the case of a spring launch a fraction of the targets will not be visible and within the same total time we will use microscanning in both filters, yielding about 1 microjansky rms in both. As the mean brightness of a normal spiral in the LW 2 filter should be about 30 microjansky per arc second, it will be possible to detect the outer regions. We will concatenate the observations as much as possible in order to get a better stabilization of the detectors. The duration of a chain of observation will always be less than 1 hour in order to ease scheduling in this difficult region. The visibility for the two possible launch periods has been calculated for each galaxy. There is little problem for an autumn launch (visibility always larger than 6.8), but a number of galaxies have visibilities lower than 4.9 for a spring launch and have been dropped; none could be added as we are working on a well-defined, complete sample. Priority 1 is given to galaxies brighter than 14.5 (mainly spirals), and priority 2 to the fainter ones (mainly irregulars and blue compacts). The priority setting is different (for good reasons!) in the corresponding PHOT proposal.