Botnets pose a serious threat to the health of the Internet. Most current network-based botnet detection systems require deep packet inspection (DPI) to detect bots. Because DPI is a computational costly process, such detection systems cannot handle large volumes of traffic typical of large enterprise and ISP networks. In this paper we propose a system that aims to efficiently and effectively identify a small number of suspicious hosts that are likely bots. Their traffic can then be forwarded to DPI-based botnet detection systems for fine-grained inspection and accurate botnet detection. By using a novel adaptive packet sampling algorithm and a scalable spatial-temporal ow correlation approach, our system is able to substantially reduce the volume of network trafic that goes through DPI, thereby boosting the scalability of existing botnet detection systems. We implemented a proof-of-concept version of our system, and evaluated it using real-world legitimate and botnet-related network traces. Our experimental results are very promising and suggest that our approach can enable the deployment of botnet-detection systems in large, high-speed networks.