In this paper we present a queue that has two classes of customers: readers and writers. Readers access the resource concurrently and writers access the resource serially. The queue discipline is FCFS: readers must wait until all writers that arrived earlier have completed service, and vice versa. The approximation can predict both the expected waiting times for readers and writers and the capacity of the queue. The queue can be used for the analysis of operating system and software resources that can be accessed both serially and concurrently, such as shared files. We have used the queue to analyze the performance of concurrent B-tree algorithms.