SCIENTIFIC ABSTRACT We propose to carry out with ISOPHOT a spatially resolved study of the dust distribution in the disks of different archetypes of spiral galaxies, and thereby investigate the spatial distribution of star formation - spontaneous and triggered - in relation to the optical image and to the spiral pattern, i.e. to the dynamical properties of the galaxy (density waves, bars, etc.). In order to discriminate dust components of different temperatures we will perform the observations in several colors from 25 to 160 um. OBSERVATION STRATEGY The present subproposal includes the photometric observations with P2, C100, and C200, and spectrophotometric observations of M33 with PHT-S. Because of its large extension, complete mapping is only possible for the central area (11'x11'), and the outer regimes are done with sparse maps. At 25um we use AOT P03 with aperture of 52" and oversampling factor 1. For C100 and C200 we use P32 with oversampling factors of 3/2 and 1, respectively. Integration times are 32, 16, 16, and 16 sec for the four filters 25, 60, 100, and 160um, respectively. In the outer regimes of the galaxy we will do sparse maps at 20 positions, using the AOTs P17-P19 and P37-P39, and the same integration times per position as above. Additional information for the HII regions in M33 is available from sparse maps done in project ISO4.2.4 ("Star formation in disk HII regions", pro= posed by Hippelein et al.). Four spectra will be taken from spiral arm and from interarm regimes, with integration times of 1024sec, each, plus one spectrum of the nucleus, with integration time of 256 sec. The S/N ratio per wavelength channel is of the order of 5. M33 is also target for CAM observations by Bonoli et al.. Total integration time for this subproposal is 14048sec, total performance time 21067sec (efficiency = 67%). The total performance time for both subproposals (iso42map + iso42m33) is 85920sec for autumn, 85712sec for spring (efficiency 65%).