Dynamic replication schemes for cost-effective, scalable content delivery networks (CDNs) are discussed. Based on content replication granularity, current dynamic replication approaches can be classified into entire replication (entire content is replicated) and object replication (individual objects are replicated) schemes. The drawbacks of these schemes are low replication performance and low scalability, respectively. We propose two schemes delivering replication performance close to that of object replication scheme with replication granularity close to that of entire replication scheme. The schemes aggregates objects in the origin server into multiple content groups and then replicate them on per-group granularity. The content groups are either statically created in advance or dynamically created upon replication. Simulation of the proposed scheme showed that the scheme of dynamic group creation can achieve almost the same performance as the optimal scheme of object replication. Even the scheme of static group creation provides much better performance than entire replication under skewed request distribution. The effect of the number of content groups on performance is also investigated.