This study commenced by surveying factors likely to affect the effectiveness of teaching. Subsequently the main aim became to develop an instrument for evaluating the effectiveness of expository teaching. Analysis of data obtained by means of Science Teaching Observation Schedule from 54 science lessons taught by 42 teachers in Rivers State confirms that science teaching in Nigerian schools is expository and didactic as practical work and higher-order intellectual transactions based on formulat-ing and testing hypotheses, solving of problems, and inter-pretation of data have been very limited. Four main teachers' groups representing £our main instructional styles were distinguished. Cluster 4 was found to consist of lessons coded from transcripts rather than insitu suggesting higher sensitivity to variations in style. The Science Teaching Observation Schedule on its own provides no means of evaluating expository teaching as it £ails to differentiate the clear from the confusing, the well structured from the incoherent. To appraise expository lessons using internal criteria, the Explanation Appraisal Schedule was developed. It consists of 18 positive and negative characteristics of which 1-9 are pedagogic characteristics and 10-18 expository. 0f the pedagogic characteristics three are structural, two content-related and four interaction-al, and the remaining nine expositional consisting of features of teacher talk which are almost instinctive, as against the pedagogic characteristics which display an element of choice. Cluster analysing data obtained by means of the E.A.S. from 12 science lessons transcripts, two teachers' clusters emerged: Cluster X (Soft-pedalling) and Cluster Y (Fast-pedalling). Though negative characteristics are by no means absent in Cluster X, Cluster Y is clearly identified as containing the less coherent and effective expositors. The E.A.S. is found to be an effective discriminating instrument. An analysis 0£ 490 questions asked by teachers shows that the less intellectually demanding questions based on Comprehension/Recall predominates in both clusters. Pupils are not encouraged to ask questions and the few asked appeared to be in response to the less effective teaching 0£ Cluster Y. Some recommendations to improve expository skills through training using the Explanation Appraisal Schedule and the Science Teaching Observation Schedule as skills development instruments, have been made.