In Wireless Sensor Networks (WSNs), sensor nodes are often deployed in large quantity and high density. Due to dense deployment, the sensed data are often redundant and highly correlated. Transmitting redundant data is not practical in the resource-constrained WSNs particularly from the viewpoint of energy conservation. Data aggregation offers a key solution to curtail the network loads and hence reduces the transmission energy consumption. In this work, we propose two data aggregation mechanisms (class-based and alpha-based) that aggregate and route data packet based on Received Signal Strength Indicator (RSSI). Each node maintains an RSSI value with respect to a sink node. When a data packet is generated, an aggregation identification (Agg ID) is associated with that packet. Using the RSSI and Agg ID in RSSI-Based Forwarding (RBF), we favor nodes closer to sink when aggregating and relaying a data packet. We show via simulation the performance of the proposed mechanisms in terms of normalized number of transmissions, energy consumed per data packet, network lifetime and packet loss probability. © 2013 IEEE.