Isodose distributions in a tissue‐equivalent phantom produced by fast neutrons from 50‐MeV deuterons incident on a thick beryllium target exhibit strong forward peaking, particularly for large fields. The design by use of decrement lines and the construction of polyethylene filters used to “flatten” those distributions are discussed and the results are illustrated. Also, the compromises of central‐axis attenuation versus effective filter width and of off‐axis peaking versus depth of “flattening” are discussed.