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Medium access control (MAC) is one of the most crucial issues in the design of Ethernet passive optical networks (EPONs). To prevent data from collision in the upstream direction, an EPON system must employ a MAC mechanism to arbitrate the access to the shared upstream channel and at the same time efficiently share the bandwidth of the upstream channel among all optical network units (ONUs). In this paper, two adaptive scheduling algorithms for MAC in an EPON system are presented. One is called the longest-queue-first (LQF) algorithm, which adaptively schedules the transmission order of different ONUs based on the instantaneous queue length of each ONU and polls the one with the longest queue first in each polling. The other is called the earliest-packet-first (EPF) algorithm, which adaptively schedules the transmission order based on the arrival time of the first packet waiting in each ONU queue and polls the one with the earliest packet first. It is shown through simulation results that the proposed scheduling algorithms can effectively improve the network performance in terms of packet delay compared with the most commonly-used round-robin scheduling algorithm.