IP multicast is gaining acceptance among service providers as the protocols and infrastructure mature. Yet characteristics of multicast traffic remain poorly understood. Using passive OC-12 monitors we observ e multicast traffic on links connecting aggregated customers and peer networks to our native multicast backbone network. We first refine existing traffic flow profiling methodologies via an exploration of temporal differences in multic ast packet trains. Based on this framework we collect multicast flow traces from four geographically dispersed nodes in the network over a one-month period. We present multicast-specific traffic patterns and characteristics including: pa cket and flow size distributions, packet duplication, packet fragmentation, address accumulation and address space distributions. We also analyze the distribution of sources per multicast group and the implications of our observations on the applicability of emerging single-source protocols. Analysis reveals results contrary to prevailing wisdom, including: (i) a preponderance of single-packet flows; (ii) a highly varia ble packet size distribution, with many large packets and strong modes; (iii) the existence of fragmented multicast traffic; and (iv) an insignificant number of simultaneous multiple- source groups. Based on our analysis, we recommend policies for deployment and improvements to protocol implementations.