The environment exerts a major influence on the star formation activity of spiral galaxies and on the nature and composition of their interstellar medium. Interacting galaxies are known to have enhanced star formation, while cluster galaxies are often highly deficient in atomic hydrogen. ISO will provide a major progress with the study of the Virgo cluster in the core program. But in order to have a quantitative estimate of the effect of the environment in the observed properties of Virgo objects, it is mandatory to construct a reference sample of isolated galaxies. The Virgo core program uses as a reference galaxies in the outskirts of the cluster. But we have no guarantee that they behave as isolated galaxies. A reference sample will also be useful to compare with galaxies in small groups or at cosmological distances. We propose to observe a sample of 120 galaxies drawn from the Karachenseva catalog of isolated galaxies. The sample objects will be observed in the CII line at 158 microns with the LWS. This line is the main coolant of the ISM, and probes both the photon-dominated regions associated with massive-star formation and the diffuse atomic phase. An important by-product of these observations will be the continuum spectrum of the targets between 60 and 200 microns, which will allow us to derive the cool dust content and to estimate the amount of gas in these galaxies. 25 galaxies have not been detected by IRAS and will be observed with PHOT at 60, 100 and 180 microns. In addition to the data at other wavelengths, which we have already collected or are part of ongoing observations (CO, HI, Halpha, radio continuum, optical colors, NIR) the proposed observations will allow us to estimate the dust and gas contents of isolated galaxies, their global star formation activity, the relative contributions of the molecular and atomic phases in the formation of the CII line, and to study the effect of the environment on all these properties.