Breeding  structures  of  two  tropical  rain  forest  tree species,  Altingia  excelsa  in  Java  and  Agathis borneensis in  Kalimantan  were  investigated.  Assuming  that  similarity  in  the  assortment  pattern  of  the isoperoxidase bands tells genetic relationship between trees, on the one hand, and that inbreeding  increases smaller  values  of  the  disagreement  counts,  on  the  other,  it  has  been  concluded that  inbreeding  occurs considerably in Altingia excelsa and to some extent in Agathis borneensis. Finding that trees showing very 
low disagreement counts are located close to each other, they were grouped as an assumptive family. It was found that different families were quite dissimilar with respect to isoperoxide  constitution and in several  leaf characters as well. The distance between two trees at which they can mate is estimated to be 16 to 18 meters or 16.5 meters and the area one family occupies is 200 to 250 m^, assuming that a family clump can be a breeding unit in Altingia excelsa, within which trees mate at random. Some families were distributed mixed with each other within  the mating  distance,  but  they  were  found  still  genetically differentiated  from  each  other. This 
reproductive  isolation  among  families  is  interpreted  to  be  due  to  genetic  differences  between  families  in flowering time. In Agathis borneensis,  there was no indication of family clump formation. Related trees may have been widely scattered in the forest, and the inbreeding of the species may be due to self-fertilization of individual trees and not to outcrossing between relatives.