SCIENTIFIC ABSTRACT These observations will survey a sample of approximately 100 stars between the ages of $10^6$ and $10^9$ years to determine the evolutionary characteristics of circumstellar disks. The observations will help establish: a. The timescale for dissipation or coagulation of the particulate material in the disks. b. The timescale for cessation of accretion onto the disks and the stars. c. The timescale for development of gaps in the distribution of disk matter. d. The influence of a variety of stellar and environmental characteristics on the formation and subsequent evolution of the disks. Far infrared continuum emission from these stars originates in circumstellar disks; the photospheric emission is well below the sensitivity of ISOPHOT, and so only disks will be detected. It is expected that disks will be detected around approximately 50% of the youngest stars and few if any of the older stars, depending on the rate at which disks evolve. The main goal of the proposal is to ascertain in exactly what age group the disk emission begins to disappear signaling the depletion of small particles in the disks. OBSERVATION SUMMARY Each star will be observed at 60 and 25 microns with PHT to determine the infrared excess radiation from cold and warm disk matter; supplemental ground-based observations at 10 microns of the same sample will detect the hotter, inner disk radiation. The sample size must be of order 100 objects to reach statistically meaningful conclusions; the sensitivity must be sufficient to detect passively heated disks around these stars which are optically thick at 60 microns, typical of young disks. The required sensitivity is achieved after approximately 1 minute of integration time at both wavelengths. Observations of nearby sky positions will allow us to subtract ambient background emission from galactic cirrus and zodiacal light. All stars have been checked for cirrus emission from the IRAS database, and the objects have been selected to minimize this emission. We expect to detect flux densities of approximately 20 mJy at 60 microns and 10 mJy at 25 microns. In most cases, the sensitivity at 60 microns will be limited by background subtraction and at 25 microns by detector noise. All stars are in compact clusters and may, therefore, be observed as concactenated observations in sparse maps (PHT17, PHT18, & PHT19). Concatenation increases the overall observing efficiency from a few percent to over 50%. Observation of a series of stars consists of observing each star and a nearby sky position in a single filter (for example, 60 microns) with 64 seconds of integration time, changing the filter (to 25 microns, in this example), and reobserving all stars and sky positions in reverse order. All observations will be made with an aperture of 52 seconds of arc.