Water-insoluble aerosol particles larger than 2 μ diameter in cirrus and other types of iced clouds were studied according to three classes: nonmagnetic particles, nonspherical magnetic particles, and magnetic spherules. Their respective concentrations were 107, 105, and 7700 particles per gram of ice. Although the concentrations of particles varied considerably within the cloud, generally similar size distributions were found for all three classes at various locations within a cloud. Particle concentration generally decreased with increasing particle size. The different size distribution of magnetic spherules found on some occasions indicates that their origin differs from that of other classes. For further study, magnetic spherules were divided into four subclasses according to shape (spherical and oblong) and surface texture (smooth and rough). Though the average abundance of each subclass of spherules present in cirrus is similar to that in air, very large differences may occur on some occasions. Chemical determinations of Ca, Na, Fe, and Ni were made for the nonmagnetic (nm) and magnetic (m) particles. Nickel was found in only 4 of 32 samples. Generally, the concentrations and ratios of elements were as follows: Fe, nm>m; Ca, m>nm; Na, m>nm; Fe/Ca, nm>m; Fe/Na, nm>m; and Ca/Na, m>nm. Magnetic aerosols (excluding spherules) are rich in calcium, indicating that they are of terrestrial origin. A pronounced separation of different types of particles, and different Ca/Na ratios, depending on altitude, were found on some occasions within a cirrus cloud. Cirrus-induced rains yielded a maximum concentration of 2000 magnetic spherules per liter of rain in some of the samples collected in Colorado, and 5000 per liter in Minnesota. The preferential transfer of magnetic particles into the ice phase of cirrus clouds affects the estimated flux of extraterrestrial spherules based on sampling at the earth's surface.