ABSTRACTExperimental and field observations are given on the biology ofPemphigus (Pemphiginus) populiCourchet with a description of the secondary host morphs from the roots ofMelilotus altissima, Lathyrus pratensisandMedicago lupulina.Twelve species ofPemphigusHartig are recorded for Britain in three subgenera:Pemphiguswith nine species,borealisTullgren,bursarius L.,filaginis(B. de F.),gairiStroyan,phenaxBörner and Blunck,pvotospiraeLicht.,rubiradicisTheob.,saliciradicis(Börner) andspirothecaePass.;Parathecabiuswith two species,lysimachiae(Börner) andauriculae(Murray);Pemphiginuswith one speciespopuliCourchet. A key to the alate migrants, alate sexuparae and apterous alienicolae is given with notes on the biology of all the species exceptborealisandrubiradic