Two Merino lines were established in 1988 and were subjected to divergent selection for maternal ranking values on lambs weaned/ ewe joined (Lw/Ej). Replacements with high values were selected in the Reproduction `+` line, while progeny of low ranking ewes was included in the Reproduction ‘–’ line. The number of ewes lambed/ewe joined was higher (P ≤ 0.10) in the ‘+’ line than in the ‘–’ line during 1989, 1991 and 1992. The proportion of multiple lambs in the `+` line was higher (P ≤ 0.05) than in the `–` line when pooled for the period 1990-1992 (0.515 vs. 0.425). This improvement in multiple births was accomplished without a reduction in lamb survival, which was higher (P ≤ 0.10) in `+` line lambs than in ‘–’ line lambs over the same period (0.776 vs. 0.712). When percentage deviations for Lw/Ej in `+` line ewes from contemporaries in the line were regressed on lambing year, an increase of 11.6 (R2 = 0.87) % per year was found. A large portion of this improvement was attributed to a change in flock structure through the replacement of unselected ewes by selected young ewes in the two breeding flocks.