The present invention pertains to a reactor tube comprising a fixed bed of Fischer-Tropsch catalyst particles, wherein the catalyst particles in 5% to 40% of the fixed bed volume at the upstream end have an average outer surface to volume ratio (S/V) in the range of between 3.0 to 4.5 mm−1, and the catalyst particles in the remaining fixed bed volume have an average outer surface to volume ratio (S/V) in the range of between 4.5 to 8.0 mm−1, and wherein the difference between the average S/V of the particles at the upstream end and the average S/V of the particles in the remaining fixed bed volume is at least 0.5 mm−1.