Di agnosti c advances have made i t possi bl e to use ul tr asonogr aph to assess pl acentati on and ther efor e zygosi ty i n uter o i n the case of monochor i oni c-monozygoti c tw i ns. Foetal behavi our of 15 monozygotic and 15 unlike-sexed dizygotic twin pairs was studi ed ser i al l y w i th ul tr asounds fr om 10 to 22 w eek s gestati onal age. Each tw i n, r egar dl ess of i ts zygosi ty, show ed i ndi vi dual i sed behavi our al styl es. One tw i n w as found to be 'domi nant' i n the

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... ense of bei ng mor e acti ve, but l ess r eacti ve, possi bl y due to the few er sti mul i bei ng gener ated by i ts co-tw i n. M onozygoti c tw i ns, as opposed to dizygotic twins, showed greater similarities in activity and reactivity levels, but were never behaviourally identical and decreased in likeness w i th i ncr easi ng age. Our data suggest that so-cal l ed i denti cal tw i ns ar e ver y si mi l ar, but not behavi our al l y i denti cal , fr om ver y ear l y i n pr egnancy. The unequal l y shar ed i ntr auter i ne envi r onment contr i butes to putti ng each monozygoti c tw i n on a pr ogr essi vel y di sti nct behavi our al path. Keyw or ds: monozygoti c tw i ns, di zygoti c tw i ns, foetal behavi our, tw i ns' i ntra-pai r di fferences, tw i ns' i ndi vi dual consi stenci es