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The objectives of this study were (a) to identify individual plants of Prosopis alba and P. pallida that could grow at near seawater salinities to use directly as clonal propagules and to establish seed orchard and (b) to analyse the population structure to aid in future germplasm collections for salinity tolerance in Prosopis. This hydroponic greenhouse study compared the growth and survival of 27 Prosopis families (seed from single mother trees) as a function of salinity from 10 to 45 dS m(-1). Nine P. alba families from one saline area (provenance) near Santiago del Estero, Argentina were compared to 14 P. pallida families from six provenances in Peru and to one family each of P. alba/flexuosa from Chile, P. juliflora from Senegal, P. flexuosa from Argentina and P. ruscifolia from Argentina. The mean of the P. pallida families had a significantly greater survival (61.1% vs. 41.7%) and percentage of seedlings that grew (37.4% vs. 23%) at seawater salinities of 45 dS m(-1) than P. alba. Families from P. pallida had the greatest mean growth at the highest salinity but P. alba families were ranked third and sixth from the top in growth. There were little differences in maximum height growth of the individual seedlings from these two species. For P. alba, the percentage of seedlings which survived at the highest salinity level had a lower correlation (r(2) = 0.46) with the number of seedlings which grew at that salinity level than for P. pallida (r(2) = 0.70). The correlation between mean family growth and maximum individual growth was lower for P. alba (r(2) = 0.24) than for P. pallida (r(2) = 0.49) suggesting that within P. alba, with a sufficiently high number of seedlings, it would be possible to find individuals with rapid growth rates in most families. Approximately 50 of the most rapidly growing seedlings at the highest salinity level have been repotted to be multiplied by rooting of cuttings and to serve as seed orchards for salt tolerance. Given the success in identifying rapidly growing P. alba individuals from only one provenance in Argentina, more extensive germplasm collections and testing are warranted to locate individuals with superior growth rates. Due to the previously demonstrated erect form, high biomass productivity and sweet pods in these P. pallida families in Haiti, Cape Verde, India and Peru, extensive recollections and field testing of P. pallida are warranted. (C) 2003 Elsevier Ltd. All rights reserved.