Clustering and data aggregation in wireless sensor networks improves scalability, and helps the efficient use of scarce resources. Yet, these mechanisms also introduce some security issues; in particular, aggregator nodes become attractive targets of physical destruction and jamming attacks. In order to mitigate this problem, we propose a new private aggregator node election protocol that hides the identity of the elected aggregator nodes both from external eavesdroppers and from compromised nodes participating in the protocol. We also propose a private data aggregation protocol and a corresponding private query protocol which allows the aggregators to collect sensor readings and respond to queries of the base station, respectively, without revealing any useful information about their identity to external eavesdroppers and to compromised nodes.