This thesis describes the activity of selected plants against biofilms of Pseudomonas aeruginosa strain PA14. The activity of plant extracts and subsequently purified compounds was evaluated using a stepwise separation process called bioassay-guided fractionation and used a microtitre plate based assay. Active extracts (showing more than 50 % biofilm inhibition, BFI) were further investigated for the presence of active compounds. The fractionation process involved the use of chromatographic techniques. Compounds were identified using NMR, GC-MS and LC-MS. Out of a total 129 extracts screened for antibiofilm activity using microtitre plate method, 44 extracts showed more than 50 % biofilm inhibition, whilst 85 extracts were found to increase biofilm formation. Four active extracts, (E333), (E341), (H338) and (M338) were selected for further investigation. A process of bioassay-guided fractionation was used to purify the phytochemicals present in each active extracts. This study led to the discovery of four bioactive compounds, namely (E333F1S1), (E341), (HA6) and (M338B) with antibiofilm activity against P. aeruginosa PA14. The active fraction (E333F1S1) from Ribes nigrum leaf was found to contain mixtures of alkanes such as n-nonadecane, 2-methylnonadecane, 2-methylicosane and 2 -methyloctacosane. The active extract of Sambucus nigra flower (E341) contained a mixture (2:1 ratio) of oleanolic and ursolic acid. Comparable activity was found at the same ratio (2:1) when these compounds were tested as a mixture of pure compounds. No significant difference (p <0.05) in activity compared to a positive control was observed when these compounds were combined in different ratios. Only weak antibiofilm activity was observed when each compound was tested on its own.The LC-MS analysis on Coriandrum sativum seed active fractions, (HA6) revealed the presence of putative compounds such as 10-undecenal, dodecanal, 2-hexyl furan, linalool oxide, caryophyllene oxide and 4-ethylcamphor. When linalool oxide was tested, it exhibited a strong activity but reduced activity when in mixtures. Both activities were significantly different (p <0.05) compared to positive control. Another active fraction of Coriandrum sativum seed, (M338B) showed the presence of putative compounds such as fatty acids, carboxylic acid, carboxylate and tetraone. This study led to the discovery of potential bioactive compounds from selected plant as antibiofilm agents against P. aeruginosa PA14. The bioactive compounds were from Ribes nigrum leaf (e.g. mixture of alkanes), Sambucus nigra flowers (e.g. mixture of ursolic acid and oleanolic acid) and Coriandrum sativum seeds (e.g. mixture of oxygenated monoterpenes, carboxylic acid, carboxylate, tetraone, glycerol, carbohydrates and fatty acids).