This article reports the results of 11 generations of selective breeding for isolation-induced, interfemale aggression in Mus musculus. Within-family selection was used to form two high, two low, and two unselected control lines, beginning with a population of wild-trapped mice. Selection was successful in establishing the divergent lines, so that in recent generations about 50% of high-line animals attack, as do 25% of controls and 5% of lows. Realized heritabilities for eight generations of selection were 0.12 for H1, 0.14 for H2, 0.34 for L1, and 0.46 for L2. Male aggression has apparently not shown a correlated response to selection for female aggression. Group housing reduces the aggression of female mice, but the order of the lines is maintained under either isolation or group housing. Practical problems encountered in this selection program are discussed.