Previous studies show that new particle formation takes place in the outflows of marine stratus and cumulus clouds. Here we show measurements of high concentrations of ultrafine particles, diameters (Dp) from 4 to 9 nm (N4--9), in interstitial cloud aerosol. These ultrafine particles indicate that in situ new particle formation occurs interstitially in cirrus clouds. Measurements were made at altitudes from 7 to 16 km over Florida with instruments on the WB-57F aircraft during Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiments (CRYSTAL-FACE) in July 2002. Size-resolved ice crystal particle concentrations and water vapor concentrations were measured to help identify the presence of cirrus clouds. About 72% of the in-cloud samples showed new particle formation events with the average N4--9 of 3.0 ¿ 103 cm-3, whereas about 56% of the out-of-cloud samples had events with the lower N4--9 of 1.3 ¿ 103 cm-3. The periods during which high N4--9 appeared were often associated with times of increasing ice water content (IWC) and high relative humidity with respect to ice (RHI); however, the measured N4--9 was not quantitatively correlated to IWC. The magnitude and frequency of new particle formation events seen in cirrus clouds were also higher than those previously observed in the tropical/subtropical upper troposphere in the absence of clouds. These results suggest that cirrus clouds may provide favorable conditions for particle formation, such as low temperatures, high RHI, high OH production (due to high water vapor), cloud electricity, and atmospheric convection. At present, however, particle formation mechanisms in clouds are unidentified.