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<p>Abstract</p> <p>Background</p> <p>A dramatic decline in the incidence of malaria due to <it>Plasmodium falciparum</it> infection in coastal East Africa has recently been reported to be paralleled (or even preceded) by an equally dramatic decline in malaria vector density, despite absence of organized vector control. As part of investigations into possible causes for the change in vector population density, the present study analysed the <it>Anopheles gambiae</it> s.l. sibling species composition in north-eastern Tanzania.</p> <p>Methods</p> <p>The study was in two parts. The first compared current species complex composition in freshly caught <it>An. gambiae</it> s.l<it>.</it> complex from three villages to the composition reported from previous studies carried out 2–4 decades ago in the same villages. The second took advantage of a sample of archived dried <it>An. gambiae</it> s.l<it>.</it> complex specimens collected regularly from a fourth study village since 2005. Both fresh and archived dried specimens were identified to sibling species of the <it>An. gambiae</it> s.l. complex by PCR. The same specimens were moreover examined for <it>Plasmodium falciparum</it> and <it>Wuchereria bancrofti</it> infection by PCR.</p> <p>Results</p> <p>As in earlier studies, <it>An. gambiae</it> s.s<it>.</it>, <it>Anopheles merus</it> and <it>Anopheles arabiensis</it> were identified as sibling species found in the area. However, both study parts indicated a marked change in sibling species composition over time. From being by far the most abundant in the past <it>An. gambiae</it> s.s<it>.</it> was now the most rare, whereas <it>An. arabiensis</it> had changed from being the most rare to the most common. <it>P. falciparum</it> infection was rarely detected in the examined specimens (and only in <it>An. arabiensis</it>) whereas <it>W. bancrofti</it> infection was prevalent and detected in all three sibling species.</p> <p>Conclusion</p> <p>The study indicates that a major shift in <it>An. gambiae</it> s.l<it>.</it> sibling species composition has taken place in the study area in recent years. Combined with the earlier reported decline in overall malaria vector density, the study suggests that this decline has been most marked for <it>An. gambiae</it> s.s<it>.</it>, and least for <it>An. arabiensis</it>, leading to current predominance of the latter. Due to differences in biology and vectorial capacity of the <it>An. gambiae</it> s.l. complex the change in sibling species composition will have important implications for the epidemiology and control of malaria and lymphatic filariasis in the study area.</p>