======================================================================== ==> In this proposal, more time is being requested for JVDHULST.LSB_GALS ======================================================================== In proposal JVDHULST.LSB_GAL, which was awarded 30000 sec ISO time, we planned to study the dust content of 20 galaxies using ISOPHOT. The plan was to observe galaxies with a large range in surface brightness (and hence in evolutionary history) at several wavelengths. As a result of the much lower in-flight sensitivity of ISOPHOT we had to revise the program and can only observe 20 galaxies at a single wavelength (160 micron). This proposal aims at recovering the lost science by observing the 20 galaxies in the original sample at a second wavelenght so that an estimate of dust temperature is possible. Spiral galaxies exhibit a large range in properties such as disk surface brightness, disk scale length, star formation rate and gas surface density. Most of the galaxies studied best so far are those with the highest surface brightness. It has become quite clear recently that galaxies with low surface brightness disks are at least as numerous as the well studied high surface brighness galaxies, and understanding their evolution is very crucial. We have good photometric data and HI imaging for a large number of galaxies over a range of 4 mag/acsec^2 in surface brightness. For the interpretation of the photometry and other properties it is important to have a good measure of the dust content of the galaxies. From our study we know that the star formation rate, the gas surface density and the metallicity of the Inter Stellar Medium (ISM) decrease with decreasing surface brightness of the disk. At the extreme end one has the genuinely low surface brightness (LSB) galaxies with very low gas surface densitites, metallicities of 0.2 - 0.5 solar and extremely low star formation rates. Little is known about how the dust properties in galaxies vary with the surface brightness of the disk. The lower surface brightness objects have not been detected in IRAS, probably because the dust is much cooler than in high surface brightness (HSB) galaxies and the gas-to-dust ratio may be down by a factor 10.