The objective of this contribution is to analyse the meaning of the expression uti frui arbitrio boni viri. This expression is contained in one of the clauses of the cautio fructuaria, which is the warranty given by the usufructuary to the owner. We can find this expression in a dozen passages of the Digest. The entire title 9 of book seven is dedicated to the cautio fructuaria, which was given in the form of a stipulatio praetoria. Uti frui arbitrio boni viri raises a question, that this paper aims to answer: does it refer to an arbitrator or to an abstract standard of behaviour? The expression arbitrium boni viri is not the only one that we fi nd in our sources. In fact there is a concurrent expression, diligens pater familias, which can be found in two passages of the Digest and in two passages of the Pauli Sententiae. I aim to understand why our sources left us two different expressions and if there is a difference between them.