The host specificity ofAmynothrips andersoniO'Neill was determined in a series of feeding behavior and starvation tests conducted in the laboratory in Argentina and Uruguay. The host plant spectrum tested involved 21 different plant species belonging to 5 families and 11 genera. A slight amount of adult feeding (1–2%) was obtained on the test plant,Polygonum aviculareL., but was not observed to occur on field plants. Observation and study of 46 plant species in nature also demonstrated an absence of thrips or their feeding. The results of these laboratory tests and observations confirm earlier observations indicating that this thrips is associated only withAlternanthera philoxeroides(Mart.) Griseb. andA. hasslerianaChod.