This paper presents a solution to counter black hole attacks in mobile ad hoc networks (MANETs). Black hole attacks are effective DoS (Denial of Service) attacks committed by fabricating routing information, attracting packets routed through them, and then dropping the packets. The ETX (Expected Transmission Count) metric is a common routing metric in MANETs. It measures the delivery ratio of a wireless link to find the high throughput routes. However, the acquisition of a ETX metric value is subject to abuse. This paper describes a solution to counter black hole attacks on the ETX metric acquisition process. The solution is called the Secure ETX (SETX) protocol. The protocol, instead of allowing individual nodes to advertise their respective delivery ratios (as in ETX) at will, allows nodes to measure neighbours' delivery ratios directly. Simulation results have shown that this novel protocol can provide a marked improvement in network performances in the presence of black hole attacks, and it can do so with a negligible level of additional overhead.